Check for musl-libc's max_align_t in addition to other variants.

Summary:
Libcxx will define its own max_align_t when it is not available. However, the
availability checks today only check for Clang's definition and GCC's
definition. In particular, it does not check for musl's definition, which is the
same as GCC's but guarded with a different macro.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: chandlerc, cfe-commits

Differential Revision: https://reviews.llvm.org/D28478

llvm-svn: 294683