tests: kernel: interrupt: workaround qemu_x86 interrupt issue

qemu_x86 seems to take an extra instruction after the sti instruction
(irq_unlock) happens before it posts the interrupts.  This can issues
if the instruction after the sti ends up reading the state that is
suppose to be updated by the ISR handler.

We see this behavior when building with LLVM.  To workaround this issue
we add an arch_nop() to provide an extra instruction to allow the
interrupts to post.

Opened zephyrproject-rtos/sdk-ng#629 to track qemu issue.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>