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

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

Actually, maybe we should split this header into `math_support` and `bit_support`. WDYT?
`clz` functions are actually in "include/llvm/ADT/bit.h"
https://github.com/llvm/llvm-project/blob/f85e7ab03534842d57d8fd385617a8d56b31bf09/llvm/include/llvm/ADT/bit.h#L280-L284

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