s390x: [wasm] Adding r3 to GP return register list

Taking similar approach as ia32 which also has 1 return register
eax (as per ia32's ABI) but uses edx as return register as well.
This will fix some failures on s390x where a function returns 2
values.

R=titzer@chromium.org, bmeurer@chromium.org

BUG=
LOG=N

Review-Url: https://chromiumcodereview.appspot.com/2426233002
Cr-Commit-Position: refs/heads/master@{#40439}

src/compiler/wasm-linkage.cc

@@ -131,7 +131,7 @@ LinkageLocation stackloc(int i, MachineType type) {
 // == s390x ==================================================================
 // ===========================================================================
 #define GP_PARAM_REGISTERS r2, r3, r4, r5, r6
-#define GP_RETURN_REGISTERS r2
+#define GP_RETURN_REGISTERS r2, r3
 #define FP_PARAM_REGISTERS d0, d2, d4, d6
 #define FP_RETURN_REGISTERS d0, d2, d4, d6