ArmPkg: prepare 32bit ARM build of StandaloneMmPkg

ArmPkg: prepare 32bit ARM build of StandaloneMmPkg

Changes in ArmPkg to prepare building StandaloneMm firmware for
32bit Arm architectures.

Adds ArmmmuStandaloneMmLib library to the list of the standard
components build for ArmPkg on when ARM architectures.

Changes path of source file AArch64/ArmMmuStandaloneMmLib.c
and compile it for both 32bit and 64bit architectures.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>