[libc] Define __UTS_NAME_LENGTH for __APPLE__

Before https://github.com/llvm/llvm-project/commit/cae84d8acf5ec7a8f7b9fd2ce33397bcc22c590c all __linux__ checks were incorrectly __unix__ checks. __unix__ being true on macOS systems therefore meant that macOS would use 65 as __UTS_NAME_LENGTH.

This commit correctly specifices __UTS_NAME_LENGTH to match XNU as 256.
https://opensource.apple.com/source/xnu/xnu-201/bsd/sys/utsname.h.auto.html

Reviewed By: thesamesam

Differential Revision: https://reviews.llvm.org/D157824