[libc++][AIX] Use C++ overloads from libc++'s math.h

AIX's system header provides these C++ overloads for compatibility with
older XL C++ implementations, but they can be disabled by defining
__LIBC_NO_CPP_MATH_OVERLOADS__ since AIX 7.2 TL 5 SP 3.

Since D109078 landed clang will define this macro when using libc++ on
AIX and we already run the lit tests with it too. This change will
enable the overloads in libc++'s math.h and we'll continue to require
the compiler to define the macro going forward.

Reviewed By: ldionne, jsji, EricWF

Differential Revision: https://reviews.llvm.org/D102172



co-authored-by: Jason Liu <jasonliu.development@gmail.com>