cmd/gc: re-enable IfacePointerOnly

CL 130240043 did this but broke ARM, because
it made newErrorCString start allocating, so we rolled
it back in CL 133810043.

CL 133820043 removed that allocation.

Try again.

Fixes #8405.

LGTM=bradfitz, dave
R=golang-codereviews, bradfitz
CC=dave, golang-codereviews, r
https://golang.org/cl/133830043

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) {