FROMLIST: arm64: preserve x18 when CPU is suspended

Don't lose the current task's shadow stack when the CPU is suspended.

Bug: 145210207
Change-Id: I8db58daadcc15a00a6f585580a3c97905c678eb3
(am from https://lore.kernel.org/patchwork/patch/1149059/

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