BeagleBoardPkg: move to ARM version of CpuExceptionHandlerLib

BeagleBoardPkg: move to ARM version of CpuExceptionHandlerLib

Change our resolution for the previously unused CpuExceptionHandlerLib
from the null implementation to the newly added implementation specific
to AARCH64 and ARM. This is needed since our CpuDxe will start using it
in a subsequent patch.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Eugene Cohen <eugene@hp.com>