Tests in abitest.c now work on Win32.

I expect that Linux-x86 is probably fine. All other architectures
except ARM are definitely broken since I haven't yet implemented
gfunc_sret for these, although replicating the current behaviour
should be straightforward.

arm-gen.c

@@ -800,6 +800,19 @@ int assign_fpreg(struct avail_regs *avregs, int align, int size)
 }
 #endif
 
+/* Return 1 if this function returns via an sret pointer, 0 otherwise */
+ST_FUNC int gfunc_sret(CType *vt, CType *ret, int *align) {
+    size = type_size(vt, &align);
+    if (size > 4) {
+        return 1;
+    } else {
+        *align = 4;
+        ret->ref = NULL;
+        ret->t = VT_INT;
+    }
+    return 0;
+}
+
 /* Generate function call. The function address is pushed first, then
    all the parameters in call order. This functions pops all the
    parameters and the function address. */