arm: vmexit: clean up ipi test and move wfi

Clean up the IPI test, because the loop_test does not need to know
anything specific about the IPI test, this is done in the ipi_test
funciton itself.

Also move the wfi() function definition to processor.h to avoid an
unused warning.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>