cortex-m: Clear pending mpu fault during mpu fault

This is a strange one: The printing code pushes a floating point
register, and is called during the mpu falt. If the floating point
registers are lazily stacked, this fp push can cause another mpu
fault to be pending during the current mpu fault, and tail chained
without returning to PendSV. Since we're already cleaning up the
fp execption reason, we might as well also clean up thisp pending,
spurious mpu exception.

Signed-off-by: Jimmy Brisson <jimmy.brisson@linaro.org>