bsd-user: start to move target CPU functions to target_arch*

Move the CPU functions into target_arch_cpu.c that are unique to each
CPU. These are defined in target_arch.h.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>