[libcxxabi] Define _LIBCXXABI_WEAK properly for mingw compilers

Copy over the compiler detection structure from libcxx, and set
_LIBCXXABI_WEAK like _LIBCPP_WEAK is set in libcxx.

This allows users to override operator new/delete, if using those
operators from libcxxabi instead of from libcxx.

Differential Revision: https://reviews.llvm.org/D89863