riscv: enable the Kconfig prompt of STRICT_KERNEL_RWX

Due to lack of hardware breakpoint support, the kernel option
CONFIG_STRICT_KERNEL_RWX should be disabled when using KGDB. However,
CONFIG_STRICT_KERNEL_RWX is always enabled now. Therefore, select
ARCH_OPTIONAL_KERNEL_RWX_DEFAULT to enable CONFIG_STRICT_KERNEL_RWX
by default, and then select ARCH_OPTIONAL_KERNEL_RWX to enable the
Kconfig prompt of CONFIG_STRICT_KERNEL_RWX so that users can turn it off.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>