Check the assembly

[thejpster]

Rust has this weird quirk where #[inline] functions are not compiled until they are used. Which means any inline-assembly code inside them isn't checked for correctness unless a function is used. For a library exporting a bunch of thin wrappers around inline-assembly, this is an issue.

This PR adds a feature to cortex-ar called check-asm, and if set, it disables the #[inline] attribute, forcing the functions to be checked.

Currently (and by default), we can build this for ARMv4T:

$ cargo +nightly build --target=armv4t-none-eabi -Zbuild-std                     
   Compiling cortex-ar v0.3.0 (/home/jonathan/Documents/github/rust-embedded/cortex-ar-patches/cortex-ar)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.45s

However, with this new feature, we see that a bunch of the barrier instructions are not available on that architecture:

$ cargo +nightly build --target=armv4t-none-eabi -Zbuild-std  --features=check-asm
   Compiling cortex-ar v0.3.0 (/home/jonathan/Documents/github/rust-embedded/cortex-ar-patches/cortex-ar)
error: instruction requires: data-barriers
  --> cortex-ar/src/asm.rs:14:27
   |
14 |         core::arch::asm!("dmb", options(nostack, preserves_flags));
   |                           ^^^
   |
note: instantiated into assembly here
  --> <inline asm>:1:2
   |
 1 |     dmb
   |     ^

error: instruction requires: data-barriers
  --> cortex-ar/src/asm.rs:30:27
   |
30 |         core::arch::asm!("dsb", options(nostack, preserves_flags));
   |                           ^^^
   |
note: instantiated into assembly here
  --> <inline asm>:1:2
   |
 1 |     dsb
   |     ^

error: instruction requires: data-barriers
  --> cortex-ar/src/asm.rs:43:27
   |
43 |         core::arch::asm!("isb", options(nostack, preserves_flags));
   |                           ^^^
   |
note: instantiated into assembly here
  --> <inline asm>:1:2
   |
 1 |     isb
   |     ^

error: could not compile `cortex-ar` (lib) due to 3 previous errors

[jonathanpallant]

Ah, I think #67 broke things and we didn't notice.

[thejpster]

You should take this commit-by-commit because the last one is just a huge number of blank lines I added because they were missing.