StandaloneMmPkg: build for 32bit arm machines

This change allows to build StandaloneMmPkg components for 32bit Arm
StandaloneMm firmware.

This change mainly moves AArch64/ source files to Arm/ side directory
for several components:  StandaloneMmCpu, StandaloneMmCoreEntryPoint
and StandaloneMmMemLib. The source file is built for both 32b and 64b
Arm targets.

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

StandaloneMmPkg/Library/StandaloneMmCoreHobLib/StandaloneMmCoreHobLib.inf

@@ -22,7 +22,7 @@
   LIBRARY_CLASS                  = HobLib|MM_CORE_STANDALONE
 
 #
-#  VALID_ARCHITECTURES           = X64 AARCH64
+#  VALID_ARCHITECTURES           = X64 AARCH64 ARM
 #
 [Sources.common]
   Common.c
@@ -30,9 +30,9 @@
 [Sources.X64]
   X64/StandaloneMmCoreHobLib.c
 
-[Sources.AARCH64]
-  AArch64/StandaloneMmCoreHobLib.c
-  AArch64/StandaloneMmCoreHobLibInternal.c
+[Sources.AARCH64, Sources.ARM]
+  Arm/StandaloneMmCoreHobLib.c
+  Arm/StandaloneMmCoreHobLibInternal.c
 
 [Packages]
   MdePkg/MdePkg.dec