aarch64: Add an aarch64_nat_target mixin class.

aarch64: Add an aarch64_nat_target mixin class.

This class includes platform-independent target methods for hardware
breakpoints and watchpoints using routines from
nat/aarch64-hw-point.c.

stopped_data_address is not platform-independent since the FAR
register holding the address for a breakpoint hit must be fetched in a
platform-specific manner.  However, aarch64_stopped_data_address is
provided as a helper routine which performs platform-independent
validation given the value of the FAR register.

For tracking the per-process debug register mirror state, use an
unordered_map indexed by pid as recently adopted in x86-nat.c rather
than a manual linked-list.