ArmPkg: CpuDxe: don't track interrupt state in a global variable

Update the CpuDxe driver to remove an assumption that it is the only
component modifying interrupt state since this can be done through BaseLib
as well. Instead of using a global variable for last interrupt state we
now check the current PSTATE value directly.

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