PrmPkg: Add details on AArch64 build to the Readme.

Specify how to build the PrmPkg for the AArch64 architecture.
Make the 2 following notes:
 - the PrmPkg has only been tested on AArch64 using the GCC5
toolchain.
 - All symbols to be listed in the PRMT as well as the
PrmModuleExportDescriptor must be explicitly preserved by resorting to
the --require-defined linker flag.

Signed-off-by: Jose Marinho <jose.marinho@arm.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

PrmPkg/Readme.md

@@ -52,6 +52,11 @@ To build `PrmPkg` as a standalone package:
      * ``. edksetup.sh``
 
 5. Build PrmPkg \
+
+The PrmPkg can be built targetting the IA32/X64 and AArch64 architectures.
+
+ - IA32/X64
+
    ``build -p PrmPkg/PrmPkg.dsc -a IA32 -a X64``
    > ***Note***: Due to the way PRM modules are compiled with exports, **only building on Visual Studio compiler tool
    chains has been tested**.
@@ -63,6 +68,12 @@ To build `PrmPkg` as a standalone package:
 > You can add your own PRM modules into the build and check them with the `PrmInfo` UEFI application described
 > later in this document and dump the PRMT table in the OS to check if your PRM module is represented as expected.
 
+ - AArch64
+    ``build -p PrmPkg/PrmPkg.dsc -a AARCH64 -t GCC5``
+
+   > ***Note***: Only builds with the GCC5 toolchain have been tested.
+   > ***Note***: For builds with the GCC5 toolchain, the PrmModuleExportDescriptor and any other handler entry points symbols, to be listed in the PRMT, must be explicitly preserved by enumerating these in the AARCH64 linker flags. The --require-defined linker flag must be used for each symbol to be preserved.
+
 ### PRM Platform GUID
 
 **IMPORTANT** PRM has a concept of a "Platform GUID" which associates a specific platform with a set of PRM modules