runtime: fix windows/arm signal handling assembly

Bug introduced in CL 288799: R12 is used but not set.

Fixes windows/arm builder.

Change-Id: I015a5a83cfa3bdd23da1ffb73713623764f2f817
Reviewed-on: https://go-review.googlesource.com/c/go/+/295109
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/sys_windows_arm.s

@@ -160,6 +160,11 @@ g0:
 	BL	(R7)		// Call the go routine
 	MOVW	16(R13), R4	// Fetch return value from stack
 
+	// Save system stack pointer for sigresume setup below.
+	// The exact value does not matter - nothing is read or written
+	// from this address. It just needs to be on the system stack.
+	MOVW	R13, R12
+
 	// switch back to original stack and g
 	MOVW	24(R13), R13
 	MOVW	20(R13), g