aarch64: Rename some of GAS's REG_TYPE_* macros

aarch64: Rename some of GAS's REG_TYPE_* macros

In GAS, the vector and predicate registers are identified by
REG_TYPE_VN, REG_TYPE_ZN and REG_TYPE_PN.  This "N" is obviously
a placeholder for the register number.  However, we don't use that
convention for integer and FP registers, and (more importantly)
SME2 adds "predicate-as-counter" registers that are denoted PN.

This patch therefore drops the "N" suffix from the existing
registers.  The main hitch is that Z was also used for the
zero register in things like R_Z, but using ZR seems more
consistent with the SP-based names.