s390: [wasm] Int64 lowering for return values

Port 776d6e9d

R=rossberg@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com, jbarboza@ca.ibm.com
BUG=
LOG=N

Change-Id: I62e59ba70fac2627a5ee00fd3007766c7c570ba3
Reviewed-on: https://chromium-review.googlesource.com/742694Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#49038}

src/compiler/wasm-linkage.cc

@@ -119,7 +119,7 @@ LinkageLocation stackloc(int i, MachineType type) {
 // == s390 ===================================================================
 // ===========================================================================
 #define GP_PARAM_REGISTERS r6, r2, r3, r4, r5
-#define GP_RETURN_REGISTERS r2, r3
+#define GP_RETURN_REGISTERS r2, r3, r4
 #define FP_PARAM_REGISTERS d0, d2
 #define FP_RETURN_REGISTERS d0, d2