PrmPkg: Add details on AArch64 build to the Readme.

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>