gc: allow ~ in import paths

Windows has paths like C:/Users/ADMIN~1. Also, it so happens
that go/parser allows ~ in import paths. So does the spec.
Fixes the build too.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5777073

src/cmd/gc/subr.c

@@ -3639,7 +3639,7 @@ isbadimport(Strlit *path)
 			yyerror("import path contains space character: \"%s\"", path->s);
 			return 1;
 		}
-		if(utfrune("!\"#$%&'()*,:;<=>?[]^`{|}~", r)) {
+		if(utfrune("!\"#$%&'()*,:;<=>?[]^`{|}", r)) {
 			yyerror("import path contains invalid character '%C': \"%s\"", r, path->s);
 			return 1;
 		}