runtime/cgo: restore the g pointer correctly in crosscall_s390x

R13 needs to be set to g because C code may have clobbered R13.

Fixes #16006.

Change-Id: I66311fe28440e85e589a1695fa1c42416583b4c6
Reviewed-on: https://go-review.googlesource.com/23910Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/cgo/gcc_s390x.S

@@ -21,9 +21,12 @@ crosscall_s390x:
 	stdy	%f4, 144(%r15)
 	stdy	%f6, 152(%r15)
 
-	/* assumes this call does not clobber r2 or r15 */
+	/* set r0 to 0 */
 	xgr	%r0, %r0
 
+	/* restore g pointer */
+	lgr	%r13, %r3
+
 	/* grow stack 8 bytes and call fn */
 	agfi    %r15, -8
 	basr    %r14, %r2