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>

arch/riscv/Kconfig

@@ -23,6 +23,8 @@ config RISCV
 	select ARCH_HAS_SET_DIRECT_MAP
 	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_STRICT_KERNEL_RWX if MMU
+	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
+	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
 	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
 	select ARCH_WANT_FRAME_POINTERS
 	select ARCH_WANT_HUGE_PMD_SHARE if 64BIT