make this crate work on stable

[japaric]

Add a Cargo feature that moves the asm! blocks into assembly files (.s) that get built at compile time. The syscall! macro would then call those external functions via extern fn.

[lucab]

Bump. I'd definitely love to have this crate available on stable, in order to get rid of a silly libc dependency just for syscall(2).

[vks]

This can now be implemented by moving to the recently stabilized asm! macro.

[japaric]

This can now be implemented by moving to the recently stabilized asm! macro.

Happy to take a PR that does this when the feature gate has been removed in a beta release. Please do not open a PR before then.

[saethlin]

On nightly-2022-01-18 and later this crate no longer builds because llvm_asm! was removed in rust-lang/rust#92816. And asm! is available on beta since 2022-01-13.

I might take a shot at a patch, but I'm quite terrible at inline assembly. If someone has any expertise with asm! please help.

[niluxv]

I did most of the platforms where the required changes are trivial in PR #33.