[TextAPI] Add support for arm64_32

Add a new architecture definition for arm64_32. The change should allow
the new architecture arm64_32 to be recognized in several pieces of
code, TextAPI parsing one of them. llvm-lipo will also recognize the
architecture and will allow lipoing files with this architecture without
failing.

Includes a small test that the architecture is recognized by llvm-nm.

Reviewed By: cishida

Differential Revision: https://reviews.llvm.org/D99673