ArmPkg: CpuDxe: fix AArch64 interrupt read masks

The AArch64 DAIF bits are different for reading (mrs) versus writing
(msr). The bitmask definitions assumed they were the same causing
incorrect results when trying to determine the current interrupt
state through ArmGetInterruptState.

The logic for interpreting the DAIF read data using the csel instruction
was also incorrect and is fixed.

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