[AArch64] Add Host identifiers for cortex-a55, cortex-a510, cortex-a710 and cortex-x2.

I noticed these were missing, so this adds Host identifiers for
cortex-a55, cortex-a510, cortex-a710 and cortex-x2, taken from their
respective TRMs.

Differential Revision: https://reviews.llvm.org/D138497