target/ppc: Implement attn instruction on BookS 64-bit processors

target/ppc: Implement attn instruction on BookS 64-bit processors

attn is an implementation-specific instruction that on POWER (and G5/
970) can be enabled with a HID bit (disabled = illegal), and executing
it causes the host processor to stop and the service processor to be
notified. Generally used for debugging.

Implement attn and make it checkstop the system, which should be good
enough for QEMU debugging.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>