Fix a number of tests to work correctly with 32/64-wide targets.

Still to be reviewed/fixed: tests/test-*, tests/[cfrs]*

tests/varying-struct-3.ispc

@@ -15,7 +15,7 @@ export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
     RET[programIndex] = g.x;
 }
 
-export void result(uniform float RET[4]) { 
+export void result(uniform float RET[]) { 
     RET[programIndex] = 15;
     RET[0] = RET[1] = 10;
 }