FROMLIST: arm64: kernel: avoid x18 in __cpu_soft_restart

The code in __cpu_soft_restart() uses x18 as an arbitrary temp register,
which will shortly be disallowed. So use x8 instead.

Bug: 145210207
Change-Id: Ifeb90566b4e41eb376fd4beaffd55fca8597aa2c
(am from https://lore.kernel.org/patchwork/patch/1149053/)
Link: https://patchwork.kernel.org/patch/9836877/


Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
[Sami: updated commit message]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>