cmd/gc: tweak interface satisfaction error message

"M requires pointer receiver" can be misinterpreted to
mean that method M should have a pointer receiver but
does not. In fact the message means "M has a pointer
receiver" (and you don't have a pointer).

Fixes #5891.

R=ken2
CC=golang-dev
https://golang.org/cl/11313043

src/cmd/gc/subr.c

@@ -1260,7 +1260,7 @@ assignop(Type *src, Type *dst, char **why)
 					"\t\thave %S%hhT\n\t\twant %S%hhT", src, dst, missing->sym,
 					have->sym, have->type, missing->sym, missing->type);
 			else if(ptr)
-				*why = smprint(":\n\t%T does not implement %T (%S method requires pointer receiver)",
+				*why = smprint(":\n\t%T does not implement %T (%S method has pointer receiver)",
 					src, dst, missing->sym);
 			else if(have)
 				*why = smprint(":\n\t%T does not implement %T (missing %S method)\n"

src/cmd/gc/typecheck.c

@@ -839,7 +839,7 @@ reswitch:
 					"\t\thave %S%hhT\n\t\twant %S%hhT", n->type, t, missing->sym,
 					have->sym, have->type, missing->sym, missing->type);
 			else if(ptr)
-				yyerror("impossible type assertion:\n\t%T does not implement %T (%S method requires pointer receiver)",
+				yyerror("impossible type assertion:\n\t%T does not implement %T (%S method has pointer receiver)",
 					n->type, t, missing->sym);
 			else if(have)
 				yyerror("impossible type assertion:\n\t%T does not implement %T (missing %S method)\n"