SecurityPkg/SecurityPkg.dsc: Add missing RngLib for ARM and RISCV64

SecurityPkg/SecurityPkg.dsc: Add missing RngLib for ARM and RISCV64

Fix SecurityPkg build breaks for ARM and RISCV64 by adding RngLib
mapping.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Daniel Schaefer <daniel.schaefer@hpe.com>