FROMLIST: arm64: reserve x18 from general allocation with SCS

Reserve the x18 register from general allocation when SCS is enabled,
because the compiler uses the register to store the current task's
shadow stack pointer. Note that all external kernel modules must also be
compiled with -ffixed-x18 if the kernel has SCS enabled.

Bug: 145210207
Change-Id: If2315ace9a879b3dd2a85f6ba43eddadc4430595
(am from https://lore.kernel.org/patchwork/patch/1149058/

)
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>