riscv: Add STACKPROTECTOR supported

The -fstack-protector & -fstack-protector-strong features are from
gcc. The patch only add basic kernel support to stack-protector
feature and some arch could have its own solution such as
ARM64_PTR_AUTH.

After enabling STACKPROTECTOR and STACKPROTECTOR_STRONG, the .text
size is expanded from  0x7de066 to 0x81fb32 (only 5%) to add canary
checking code.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/process.c

@@ -24,6 +24,12 @@
 
 register unsigned long gp_in_global __asm__("gp");
 
+#ifdef CONFIG_STACKPROTECTOR
+#include <linux/stackprotector.h>
+unsigned long __stack_chk_guard __read_mostly;
+EXPORT_SYMBOL(__stack_chk_guard);
+#endif
+
 extern asmlinkage void ret_from_fork(void);
 extern asmlinkage void ret_from_kernel_thread(void);