PrmPkg: Build Prm Samples with GCC for AARCH64

PrmPkg: Build Prm Samples with GCC for AARCH64

- Add the --prm flag to the GENFW_FLAGS
- Add the --no-gc-section to the linker flags so that apparently
unreferenced symbols are not prematurely removed from the .dll which
is used to generate the Prm module .efi.
- Force the linker to maintain the PrmModuleExportDescriptor symbol.
- Force the linker to maintain the PRM handler funtion's symbol.

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>