[MTE] Add NT_ANDROID_TYPE_MEMTAG

This ELF note is aarch64 and Android-specific. It specifies to the
dynamic loader that specific work should be scheduled to enable MTE
protection of stack and heap regions.

Current synthesis of the ".note.android.memtag" ELF note is done in the
Android build system. We'd like to move that to the compiler, and this
is the first step.

Reviewed By: MaskRay, jhenderson

Differential Revision: https://reviews.llvm.org/D119381