SecurityPkg: Add ARM/AARCH64 arch to enable RngTest module build.

SecurityPkg: Add ARM/AARCH64 arch to enable RngTest module build.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=680

Adding ARM and AARCH64 to SUPPORTED_ARCHITECTURES in SecurityPkg.dsc
to enable RngTest module build, since this is one platform-independent
application.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qin Long <qin.long@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>