[libc][NFC] Make few maths functions buildable outside of LLVM libc build.

Few math functions manipulate errno. They assumed that LLVM libc's errno
is available. However, that might not be the case when these functions
are used in a libc which does not use LLVM libc's errno. This change
switches such uses of LLVM libc's errno to the normal public errno macro.
This does not affect LLVM libc's build because the include order ensures
we get LLVM libc's errno. Also, the header check rule ensures we are only
including LLVM libc's errno.h.