cgo: implement C unions

Unions are somewhat hard to implement in Go because they are not a
native type. But it is actually possible with some compiler magic.

This commit inserts a special "C union" field at the start of a struct
to indicate that it is a union. As such a field cannot be written
directly in Go, this is a useful to distinguish structs and unions.

testdata/cgo/main.c

@@ -8,8 +8,11 @@ double globalDouble = 3.2;
 _Complex float globalComplexFloat = 4.1+3.3i;
 _Complex double globalComplexDouble = 4.2+3.4i;
 _Complex double globalComplexLongDouble = 4.3+3.5i;
-collection_t globalStruct = {256, -123456, 3.14};
+collection_t globalStruct = {256, -123456, 3.14, 88};
+int globalStructSize = sizeof(globalStruct);
 short globalArray[3] = {5, 6, 7};
+joined_t globalUnion;
+int globalUnionSize = sizeof(globalUnion);
 
 int fortytwo() {
 	return 42;
@@ -26,3 +29,17 @@ int doCallback(int a, int b, binop_t callback) {
 void store(int value, int *ptr) {
 	*ptr = value;
 }
+
+void unionSetShort(short s) {
+	globalUnion.s = s;
+}
+
+void unionSetFloat(float f) {
+	globalUnion.f = f;
+}
+
+void unionSetData(short f0, short f1, short f2) {
+	globalUnion.data[0] = 5;
+	globalUnion.data[1] = 8;
+	globalUnion.data[2] = 1;
+}