aarch64: Extend aarch64_field to support constants

aarch64: Extend aarch64_field to support constants

Many instructions have constraints on the range of registers they can
use.  This means that some bits in the register number are fixed, and
therefore aren't mapped to a field in the instruction encoding.
Currently we use various adhoc rules to handle these fixed bits, but
this doesn't handle all cases and we often have to write new code to
support new combinations of permitted registers.

This patch allows these constant bits to instead be specified in the
same structure used to represent instruction fields.  Uses of the new
constant fields will be introduced in subsequent patches.