aarch64: work around QEMU 'wfi' issue

Work around an issue where the emulator ignores host OS
signals when inside a `wfi` instruction.

This should be reverted once this has been addressed in the
AARCH64 build of QEMU in the SDK.

See https://github.com/zephyrproject-rtos/sdk-ng/issues/255



Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>