ArmPkg/ArmLib: add ArmHasGicSystemRegisters () helper function

ArmPkg/ArmLib: add ArmHasGicSystemRegisters () helper function

Create a helper function to eliminate direct feature register reading,
which gets messy in code shared between ARM/AArch64.

Returns BOOLEAN True if the CPU implements the GIC System Register
Interface (any version), otherwise returns BOOL False.

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