target-arm: A64: Implement WFI

Implement the WFI instruction for A64; this just involves wiring
up the instruction, and adding a gen_a64_set_pc_im() which was
accidentally omitted from the A64 decoder top loop.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
