[libc++] Use _BitScanForward64 more often, by fixing availability detection, avoiding calling _BitScanForward twice#142000

eugenegff wants to merge 2 commits intollvm:mainllvm/llvm-project:mainfrom

eugenegff:ryu_bitscanforward64_performance_fixeugenegff/llvm-project:ryu_bitscanforward64_performance_fixCopy head branch name to clipboard