when stable, use `core::arch::arm` to implement `cortex_m::asm`

[japaric]

this will let us provide the cortex_m::asm API on stable without depending on an external assembler.

[japaric]

Triage: blocked on cortex_m::asm being stabilized.

[japaric]

Removing from the RC1 (edition) milestone because thanks to PR #108 this crate no longer on GCC so this is just a nice to have.

[jonas-schievink]

We now use Rust's new inline assembly (rust-lang/rust#72016) and an FFI shim to implement cortex_m::asm, removing the need for any external tooling. Inline asm has higher chances of being stabilized than core::arch::arm, so maybe we should close this issue? (doesn't seem too important to keep tracking this; we can remove the FFI shim once inline asm is stable)

[adamgreig]

SGTM. I agree that inline asm getting stabilised looks more likely than all the relevant instructions landing in core::arch::arm, and in any event we need a few assembly routines for some methods anyway.