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.

Link: https://patchwork.kernel.org/patch/9836877/
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[Sami: updated commit message]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/cpu-reset.S

@@ -42,11 +42,11 @@ ENTRY(__cpu_soft_restart)
 	mov	x0, #HVC_SOFT_RESTART
 	hvc	#0				// no return
 
-1:	mov	x18, x1				// entry
+1:	mov	x8, x1				// entry
 	mov	x0, x2				// arg0
 	mov	x1, x3				// arg1
 	mov	x2, x4				// arg2
-	br	x18
+	br	x8
 ENDPROC(__cpu_soft_restart)
 
 .popsection