cgo: implement void* pointer type

void* is translated to unsafe.Pointer on the Go side.

testdata/cgo/main.c

@@ -9,6 +9,8 @@ _Complex float globalComplexFloat = 4.1+3.3i;
 _Complex double globalComplexDouble = 4.2+3.4i;
 _Complex double globalComplexLongDouble = 4.3+3.5i;
 char globalChar = 100;
+void *globalVoidPtrSet = &global;
+void *globalVoidPtrNull;
 collection_t globalStruct = {256, -123456, 3.14, 88};
 int globalStructSize = sizeof(globalStruct);
 short globalArray[3] = {5, 6, 7};