[clang] [llvm] [X86] support reserve r8~r15 on X86_64 (PR #180242)

[Fangrui Song via llvm-commits]

MaskRay wrote:

severe compile time regression for stage1-O0-g: @nikic 

https://llvm-compile-time-tracker.com/compare.php?from=1b633d6d6d75c172ecd9b9a4e250842468edb137&to=e71f327b46059fe232b205414a3e8c1fc537c80f&stat=instructions:u

https://github.com/llvm/llvm-project/pull/180242