[libc] [llvm] [NFC] rename builtin_wrapper into math_extras (PR #72998)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

FYI I intend to add the following mask functions from llvm/Support/MathExtras.h
https://github.com/llvm/llvm-project/blob/6088e9cdba902f78d88bbd3e7ff21c6ddb8fcf98/llvm/include/llvm/Support/MathExtras.h#L65-L88

https://github.com/llvm/llvm-project/pull/72998