riscv: add STRICT_KERNEL_RWX support

The commit contains that make text section as non-writable, rodata
section as read-only, and data section as non-executable.

The init section should be changed to non-executable.

Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>