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'.

all.h

@@ -357,7 +357,8 @@ struct Fn {
 
 struct Typ {
 	char name[NString];
-	int dark;
+	char isdark;
+	char isunion;
 	int align;
 	uint64_t size;
 	uint nunion;