target-arm: A64: add set_pc cpu method

When executing translation blocks we need to be able to recover
our program counter. Add a method to set it for AArch64 CPUs.
This covers user-mode, but for system mode emulation we will
need to check if the CPU is in an AArch32 execution state.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
