riscv: select ARCH_HAS_STRICT_KERNEL_RWX only if MMU

ARCH_HAS_STRICT_KERNEL_RWX is not useful for NO-MMU systems.
Furthermore, has this option leads to very large boot image files on
64bits architectures, do not enable this option to allow supporting
no-mmu platforms such as the Kendryte K210 SoC based boards.

Fixes: 00cb41d5

 ("riscv: add alignment for text, rodata and data sections")
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>