cmd/gc: change interface representation: only pointers in data word

Note that there are various cleanups that can be made if we keep
this change, but I do not want to start making changes that
depend on this one until the 1.4 cycle closes.

Fixes #8405.

LGTM=r
R=golang-codereviews, adg, r, bradfitz
CC=golang-codereviews, iant
https://golang.org/cl/130240043

src/cmd/gc/subr.c

@@ -3808,7 +3808,7 @@ isdirectiface(Type *t)
 	// where the data word can hold a pointer or any
 	// non-pointer value no bigger than a pointer.
 	enum {
-		IfacePointerOnly = 0,
+		IfacePointerOnly = 1,
 	};
 
 	if(IfacePointerOnly) {