x86_64: Support run-time detection for cmpxchg16b on pre-1.69 rustc #154
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Unlike AArch64 and RISC-V, x86's assembler doesn't check instruction requirements for the currently enabled target features. In the first place, there is no option in the x86 assembly for such case, like ARM .arch_extension, RISC-V .option arch, PowerPC .machine, etc.
(However, we set target_feature(enable) when available (Rust 1.69+) in case a new codegen backend is added that checks for it in the future, or an option is added to the assembler to check for it.)