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.h

@@ -6,11 +6,21 @@ typedef int * intPointer;
 void store(int value, int *ptr);
 
 typedef struct collection {
-	short s;
-	long  l;
-	float f;
+	short         s;
+	long          l;
+	float         f;
+	unsigned char c;
 } collection_t;
 
+typedef union joined {
+	myint s;
+	float f;
+	short data[3];
+} joined_t;
+void unionSetShort(short s);
+void unionSetFloat(float f);
+void unionSetData(short f0, short f1, short f2);
+
 // test globals
 extern int global;
 extern _Bool globalBool;
@@ -21,7 +31,10 @@ extern _Complex float globalComplexFloat;
 extern _Complex double globalComplexDouble;
 extern _Complex double globalComplexLongDouble;
 extern collection_t globalStruct;
+extern int globalStructSize;
 extern short globalArray[3];
+extern joined_t globalUnion;
+extern int globalUnionSize;
 
 // test duplicate definitions
 int add(int a, int b);