FROMLIST: arm64: efi: restore x18 if it was corrupted

If we detect a corrupted x18 and SCS is enabled, restore the register
before jumping back to instrumented code. This is safe, because the
wrapper is called with preemption disabled and a separate shadow stack
is used for interrupt handling.

Bug: 145210207
Change-Id: Idb75117e38c895231a14f5573261861e722c1264
(am from https://lore.kernel.org/patchwork/patch/1149060/

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