runtime: fix sigfwd for aix/ppc64

This commit fixes runtime.sigfwd for aix/ppc64. fn is a function
descriptor and not a function. R2 must be saved and restored.

Change-Id: Ie506b0bdde562ca37202d19973ba1d537c3d64e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/164015Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sys_aix_ppc64.s

@@ -77,8 +77,15 @@ TEXT runtime·sigfwd(SB),NOSPLIT,$0-32
 	MOVD	info+16(FP), R4
 	MOVD	ctx+24(FP), R5
 	MOVD	fn+0(FP), R12
-	MOVD	R12, CTR
+	// fn is a function descriptor
+	// R2 must be saved on restore
+	MOVD	0(R12), R0
+	MOVD	R2, 40(R1)
+	MOVD	8(R12), R2
+	MOVD	R0, CTR
 	BL	(CTR)
+	MOVD	40(R1), R2
+	BL	runtime·reginit(SB)
 	RET