Support run-time detection for cmpxchg16b on x86_64 on pre-1.69 rustc. (#154)

Make into_inner const fn on Rust 1.56+. (align to the std atomic change in Rust 1.79) (dee1f89)

Work around rustc_codegen_gcc bug on x86_64. (d938f77)

Optimize x86_64 atomics.

• Optimize 128-bit load/store on Zhaoxin CPU with AVX. (86cee8f)
• Optimize 128-bit SeqCst store on Intel/AMD/Zhaoxin CPU with AVX. (#156, 0483042)
• Remove needless test in CAS. (573e025)

Make rustc version detection robust for custom toolchains. (f8ea85e)

Respect RUSTC_WRAPPER in rustc version detection.

Our build script is now less likely to be re-run unnecessarily in versions where the cargo bug fix is available (cargo 1.79+). (52c277b)