Use new LLVM-based Xamarin.Android toolchain

[grendello]

Context: #6685
Context: fd5f31c

So far, Xamarin.Android has been using a GNU Binutils toolchain to compile and link native assembler code we generate
during application build. Unfortunately, Binutils have a problem with certain filename encodings on Windows and so we
decided to switch to an LLVM-based toolchain which handles such file names without issues.

However, since Mono/dotnet AOT expects a GNU build chain, it was necessary to implement a GNU Assembler (gas) wrapper
around the LLVM llvm-mc assembler, so that command lines used by the Mono AOT compiler keep working fine.

Since LLVM utilities are multi-target by default and, unlike GNU Binutils, they don't need separate builds of every utility,
we provide GNU Binutils architecture-prefixed wrapper scripts which invoke their LLVM counterparts with appropriate parameters.

The following LLVM utilities are included:

• llvm-mc (assembler)
• lld (linker)
• llvm-strip

[jonathanpeppers]

Might check if this fixes: #6685

And add proj.AotAssemblies = true; to that test?

[grendello]

Might check if this fixes: #6685

And add proj.AotAssemblies = true; to that test?

Will do once this run's done, I want to see if there are any obvious issues.

[grendello]

Some tests fail with the following error message:

obj/Release/aot/x86/UnnamedProject.dll/temp.s:20:2: error: unknown directive (TaskId:174)
  [aot-compiler stderr]         .hword 2

This is because llvm-mc doesn't support .hword (alias for `.short) on x86 targets. Fixing this will require changes to Mono/dotnet AOT probably

[grendello]

Mono/dotnet AOT PRs:

dotnet/runtime#64561
mono/mono#21416

[grendello]

It's ready for review, but before merging the new toolchain has to be released (it's currently a pre-release) and that depends on this PR being green.

[grendello]

The diacritics tests fails in the AOT compiler:

                     [AOT] response file obj\Release\aot\armeabi-v7a\Java.Interop.dll\response.txt: "--aot=asmwriter,mtriple=armv7-linux-gnueabi,tool-prefix=C:\a\_work\1\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\binutils\arm-linux-androideabi-,outfile=obj\Release\aot\armeabi-v7a\libaot-Java.Interop.dll.so,llvm-path=C:\a\_work\1\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android,temp-path=obj\Release\aot\armeabi-v7a\Java.Interop.dll,ld-name=ld" "C:\a\_work\1\a\TestRelease\02-11_11.05.54\temp\SmokeTestBuildWithSpecialCharactersTrueTrue\テスト\obj\Release\android\assets\shrunk\Java.Interop.dll"
                      (TaskId:217)
                     [aot-compiler stderr] The specified response file can not be read (TaskId:217)
11:19:26.258   1:7>C:\a\_work\1\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Legacy.targets(692,5): error XA3001: Could not AOT the assembly: テスト.dll [C:\a\_work\1\a\TestRelease\02-11_11.05.54\temp\SmokeTestBuildWithSpecialCharactersTrueTrue\テスト\テスト.csproj]

[grendello]

It appears the toolchain works fine, I've created its new release and bumped this PR to use it. There were a couple of test failures here which appeared not to be related to the PR (one HTTP timeout and one dotnet run failure that did however build and install the app, logcat contained no errors neither did msbuild.binlog so I have no idea what went wrong, we'll see if it fails again)

[jonpryor]

This should instead link to: dotnet/runtime#65484

[jonpryor]

I wonder if we should instead do:

public void SmokeTestBuildWithSpecialCharacters ([Values (false, true)] bool forms, [Values (false /*, true */)] bool aot)

[jonpryor]

…and then set this to aot.

[jonpryor]

Context: fd5f31cc89648957243f31ba8fd7af7fccba11e4
Context: https://github.com/xamarin/xamarin-android/issues/6685
Context: https://github.com/xamarin/xamarin-android-binutils
Context: https://github.com/xamarin/xamarin-android-binutils/commit/6d4e3bbceff617235abf03727d5233cb97b80cfb
Context: https://github.com/xamarin/xamarin-android/issues/6840

Changes: https://github.com/xamarin/xamarin-android-binutils/compare/2.35.2-XA.1...L_13.0.1-4.0.1

So far, Xamarin.Android has been using a GNU Binutils toolchain to
compile and link native assembler code we generate during application
build.  Unfortunately, Binutils have a problem with certain filename
encodings on Windows -- which required that we downgrade to Binutils
2.35.2 in commit fd5f31cc -- so we decided to switch to an LLVM-based
toolchain which handles such file names without issues.

However, as mono/mono & dotnet/runtime AOT expects a
GNU Binutils-compatible toolchain, it was necessary to implement a
GNU Assembler (`gas`) wrapper around the LLVM `llvm-mc` assembler, so
that command lines used by the Mono AOT compiler keep working fine.

Since LLVM utilities are multi-target by default and, unlike GNU
Binutils, LLVM doesn't need separate builds of every utility, we now
provide GNU Binutils architecture-prefixed wrapper scripts which
invoke their LLVM counterparts with appropriate parameters.

The following LLVM utilities are included:

  - `llvm-mc` (assembler)
  - `lld` (linker)
  - `llvm-strip`

Migrating to LLVM from GNU Binutils increases our install size:

  * macOS `.pkg` size increases by ~29MB,
    installation size increases by ~75MB.
  * Windows `.vsix` size increases by ~14MB,
    installation size increases by ~40MB.

Aside: We've updated our unit tests to set
`$(_DisableParallelAot)`=True, because when AOT is done in parallel,
it's very difficult to make sense of the AOT compiler messages, as
messages from multiple processes are all intermixed.  We're disabling
parallel AOT to preserve our sanity when things break.

TODO: the Resulting `.apk` sizes also increase unexpectedly, with
`Xamarin.Forms_Performance_Integration-Signed-Release-Profiled-Aot.apkdesc`
showing a 1.7MB increase in `.apk` size.  We believe that this is
because of more verbose debug symbols.
xamarin/xamarin-android#6840 will track this.