Teach libc++ to use native NetBSD's max_align_t

Summary:
The NetBSD headers ship with max_align_t, that is not
compatible with the fallback version in libc++.

There is no defined a compiler specific symbol in the headers like:
 - __CLANG_MAX_ALIGN_T_DEFINED
 - _GCC_MAX_ALIGN_T
 - __DEFINED_max_align_t

Sponsored by <The NetBSD Foundation>

Reviewers: chandlerc, dlj, EricWF, joerg

Reviewed By: joerg

Subscribers: bsdjhb, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D47814

llvm-svn: 340224