ArmPkg: remove duplicated ARM/AArch64 ArmGicArchSecLib sources

ArmPkg: remove duplicated ARM/AArch64 ArmGicArchSecLib sources

The ID register access was the only difference between them, so
after switching to the ArmHasGicSystemRegisters () helper, there
is no longer any need to have separate ARM/AArch64 source files
for ArmGicArchSecLib, so unify them and drop the subdirectories.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>