aarch64: Tweak parsing of integer & FP registers

aarch64: Tweak parsing of integer & FP registers

Integer registers were parsed indirectly through
aarch64_reg_parse_32_64 (and thus aarch64_addr_reg_parse) rather
than directly through parse_reg.  This was because we need the
qualifier associated with the register, and the logic to calculate
that was buried in aarch64_addr_reg_parse.

The code that parses FP registers had the same need, but it
open-coded the calculation of the qualifier.

This patch tries to handle both cases in the same way.  It is
needed by a later patch that tries to improve the register-related
diagnostics.