[AARCH64] [MC] add memtag as an alias of mte architecture extension

Add memtag as an alis of met architectture extesion to be consistent
with GNU as.

LINK:https://sourceware.org/bugzilla/show_bug.cgi?id=26339

Reviewed By: nickdesaulniers, MaskRay

Differential Revision: https://reviews.llvm.org/D85620