Eliminate build.rs-generated Aarch64 atomic macros

[qinghon]

Replace build.rs Rust generation with macros, using the unstable ${concat(...)}.

Fixes: #947

[tgross35]

Regarding #947 (comment): we should make use of the unstable ${concat(...)} (rust-lang/rust#124225). This just means writing the macro in source rather than generating it in the build script and include!ing it there

compiler-builtins/compiler-builtins/src/aarch64_linux.rs

Line 266 in add44a7

include!(concat!(env!("OUT_DIR"), "/outlined_atomics.rs"));

.

[qinghon]

@tgross35 Is it like this now?

[tgross35]

Looks good, thanks! Just two small nits

[tgross35]

Thanks for the changes, I'll double check this tomorrow and merge it.

[tgross35]

LGTM, thank you!