Merge #423

423: Swap to just-stabilised asm!() and global_asm!() macros r=thejpster a=adamgreig

Once Rust 1.59 is released in a couple of days, the `asm!()` and `global_asm!()` macros will become stable, and we will no longer require the various precompiled binaries we've used until now in cortex-m and cortex-m-rt. cc #420.

This PR uses `asm!()` in cortex-m, and removes the inline-asm feature, since I anticipate this going into cortex-m 0.8 and so we don't need to leave it behind for compatibility. In various places the previous version would call an extern C method when built for the native target, which would only fail at link time; to preserve the ability to build on x86 I've either made the whole method require the `cortex_m` configuration, or where appropriate/convenient simply skipped the `asm!()` call.

This PR replaces the old gcc-preprocessed `asm.S` in cortex-m-rt with use of `global_asm!()`, although since you can't normally use `#[cfg(...)]` attributes with `global_asm!()`, there's also a slightly scary macro modified from one invented by `@Dirbaio` for a similar purpose. I considered putting the initialisation of LR behind an armv6m flag, but since we want to restore it after calling `__pre_init` it seemed better to just leave it the same on both targets. I added Cargo features to optionally set SP and VTOR at startup, which has been variously requested but would previously have required multiplicatively more pre-built binaries. Now: no problem. Relevant issues:

* https://github.com/rust-embedded/cortex-m-rt/issues/283
* https://github.com/rust-embedded/cortex-m-rt/issues/55
* https://github.com/rust-embedded/cortex-m-rt/issues/254
* https://github.com/rust-embedded/cortex-m-rt/issues/102
* https://github.com/rust-embedded/cortex-m-rt/pull/338

I've tested these on a couple of targets (and updated the CI): on the whole there's a small improvement in code size due to everyone getting inlined asm, especially in `cortex_m::interrupt::free()`.

The major downside is we bump our MSRV from 1.42 (March 2020) to 1.59 (Feb 2022). For cortex-m, I propose putting these changes in the upcoming 0.8 release (which is technically what the master branch is already on) and not backporting. For cortex-m-rt I'm not sure: we don't have any other pending breaking changes, so we could consider a patch release. Anyway, this PR doesn't commit to any particular releases, so we can decide that later.

For cortex-m-semihosting/panic-semihosting I think a patch release would be ideal, especially since we had to yank the last c-m-sh release due to conflicting prebuilt binaries (a problem that should now vanish).

Also tagging these issues that I think might also benefit from new inline asm:
* https://github.com/rust-embedded/cortex-m/issues/265
* https://github.com/rust-embedded/cortex-m/issues/215
* https://github.com/rust-embedded/cortex-m/issues/406

Co-authored-by: Adam Greig <adam@adamgreig.com>

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