flag types defined as unions

The risc-v abi needs to know if a
type is defined as a union or not.

We cannot use nunion to obtain this
information because the risc-v abi
made the unfortunate decision of
treating

	union { int i; }

differently from

	int i;

So, instead, I introduce a single
bit flag 'isunion'.

amd64/sysv.c

@@ -77,7 +77,7 @@ typclass(AClass *a, Typ *t)
 	a->size = sz;
 	a->align = t->align;
 
-	if (t->dark || sz > 16 || sz == 0) {
+	if (t->isdark || sz > 16 || sz == 0) {
 		/* large or unaligned structures are
 		 * required to be passed in memory
 		 */