AArch64: Rename A64_/A32_ register prefix to x/w.

A64/A32 look like architecture name, but they are all for arm64.
Use lower-case to name the registers defined in "ARM ARM" which can
also be directly used in assembly file.
Use upper-case to name the registers which are other aliases.

Change-Id: I0ac38ed75f977fdc362288b01179b84feaee5614