[docs][AArch64] Label Features with Arm ARM Names

This patch adds the names of the Arm Architecture Reference Manual (ARM)
features to the corresponding Subtarget Features in the AArch64 backend
and target parser.

The aim of this is to make it clearer what architectural features a
subtarget feature might enable (so, which features a CPU must provide to
support that subtarget feature), and so make it easier to add new CPUs
in the future.

Differential Revision: https://reviews.llvm.org/D131257