Hopefully good fix for va_listval build fails (re: 41ed8047, etc.)

The last commit still failed to build on macOS M1. That va_listval
macro keeps causing trouble. It's an AST thing that is defined in
src/lib/libast/features/common. That looks like some incredibly
opaque attempt to make it compatible with everything, and clearly
it no longer works properly on all systems. I don't dare touch it,
though. That code looks like any minimal change will probably break
the build on some system or other.

src/lib/libast/features/hack:
- Add feature test to check if that macro needs (0) no workaround,
  or (1) the workaround from the 93v- beta, or (2) the FreeBSD one.
  Whichever version compiles first, it will use. If none does, this
  test will not output a value, which will be treated as 0.

src/lib/libast/hash/hashalloc.c,
src/lib/libast/string/tokscan.c:
- Update to use the result of the hack feature test.

src/lib/libast/Mamfile:
- Update for new #include dependencies.

src/lib/libast/hash/hashalloc.c

@@ -31,6 +31,7 @@
 static const char id_hash[] = "\n@(#)$Id: hash (AT&T Research) 1996-08-11 $\0\n";
 
 #include "hashlib.h"
+#include "FEATURE/hack"
 
 Hash_info_t	hash_info = { 0 };
 
@@ -152,10 +153,13 @@ hashalloc(Hash_table_t* ref, ...)
 				va_copy(*vp, ap);
 				vp++;
 			}
-#if __clang__ && __SIZEOF_POINTER__ == 4 && !__APPLE__
+#if _need_va_listval_workaround
 			{
+#   if _need_va_listval_workaround == 2
+				va_listarg	np;
+#   else
 				va_list		np;
-
+#   endif
 				np = va_listval(va_arg(ap, va_listarg));
 				va_copy(ap, np);
 			}