riscv: Set ARCH_HAS_STRICT_MODULE_RWX if MMU

Now we can set ARCH_HAS_STRICT_MODULE_RWX for MMU riscv platforms, this
is good from security perspective.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>