* Allow FFI-unsafe warnings for u128/i128
Handle new warnings on nightly, and we shouldn't need to worry about
these with compiler-builtins since this is tied to a particular compiler.
* Clean up crate attributes
* No need for stability marker
* Rustdoc docs not used for this crate
* Remove old build-system related cruft from rustc itself.
* Run `cargo fmt`
This commit fixes a bug accidentally introduced in #285 where some
lingering references remained to `#[cfg(thumbv6m)]` but this, since the
historical revert, was renamed to `#[cfg(thumb_1)]`. This caused on the
thumbv6m platform for the intrinsics to be accidentally omitted because
the build script didn't actually compile them but the Rust code thought
the C code was in use.
After correcting the `#[cfg]` statements the CI configuration for the
`thumb*` family of targets was all updated. The support for xargo
testing was removed from `run.sh` since it had long since bitrotted, and
the script was updated to simply build the intrinsics example to attempt
to link for each of these targets. This in turn exposed the bug locally
and allowed to confirm a fix once the `#[cfg]` statements were
corrected.
cc rust-lang/rust#60782
It seems that the intrinsics that were generated for the functions in
example/intrinsics.rs where different implementations were given for
thumb6m-none-eabi target, have now been implemented in Rust so
configuration is not needed anymore.
