arm64: Add gcc Shadow Call Stack support

arm64: Add gcc Shadow Call Stack support

Shadow call stacks will be available in GCC >= 12, this patch makes
the corresponding kernel configuration available when compiling
the kernel with the gcc.

Note that the implementation in GCC is slightly different from Clang.
With SCS enabled, functions will only pop x30 once in the epilogue,
like:

   str     x30, [x18], #8
   stp     x29, x30, [sp, #-16]!
   ......
-  ldp     x29, x30, [sp], #16	  //clang
+  ldr     x29, [sp], #16	  //GCC
   ldr     x30, [x18, #-8]!

Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=ce09ab17ddd21f73ff2caf6eec3b0ee9b0e1a11e
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220303074323.86282-1-ashimida@linux.alibaba.com