Refactor code slightly in nat/aarch64-linux-hw-point.c (aarch64_linux_get_debug_reg_capacity)

Since the two locations which check the debug arch are the same code currently, it is
a good idea to factor it out to a new function and just use that function from
aarch64_linux_get_debug_reg_capacity. This is also the first step to support
ARMv8.4 debug arch.