[DRAFT] Write barrier without any RWX pages

[davidwrighton]

Add a new config switch DOTNET_UseGCWriteBarrierCopy to control whether or not to use a copy of the WriteBarrier code instead of assembly code embedded in the coreclr binary. Typically we do this to improve performance, but this change enables a new path where we just have the barrier as assembly code in the coreclr binary. Also, with this change, the 1 copy of the code which is used for this scenario is shared between NativeAOT and coreclr.

DRAFT Because...

• The DOTNET_UseGCWriteBarrierCopy flag is set to 0 by default now, but when this is checked in it should be set to 1
• Linux-X86 builds are broken
• Other builds are probably broken (WinX64 and WinArm64 are known to build)
• Testing of scenarios like GC shadow has not been done
• Testing of general GC behavior has not been done
• Functional correctness has not been tested (WinX64 is known to not crash on helloworld)

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[filipnavara]

FWIW this would not work anyway... we compile all code on Apple platforms as .subsections_via_symbols and the Apple linker is free to reorder the functions (or discard unused ones if told to). Compiling without subsections_via_symbols is not an option due to various bugs in different linker versions. The only other option to keep the desired order is to mark the successive symbols as .alt_entry, which also happens to run into linker bugs if not done extremely carefully...