target-arm: Move aarch64_cpu_do_interrupt() to helper.c

target-arm: Move aarch64_cpu_do_interrupt() to helper.c

Move the aarch64_cpu_do_interrupt() function to helper.c. We want
to be able to call this from code that isn't AArch64-only, and
the move allows us to avoid awkward #ifdeffery at the callsite.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>