Improve name mangling for gdb

Remove __extensions__ from method symbols as well as the meth_XXX. The XXX is
now used to append a few characters at the end of the name of the symbol.

Closes #6602

src/test/compile-fail/ambig_impl_2_exe.rs

@@ -15,6 +15,6 @@ use ambig_impl_2_lib::me;
 trait me {
     fn me(&self) -> uint;
 }
-impl me for uint { fn me(&self) -> uint { *self } } //~ NOTE is `__extensions__::me`
+impl me for uint { fn me(&self) -> uint { *self } } //~ NOTE is `me$uint::me`
 fn main() { 1u.me(); } //~ ERROR multiple applicable methods in scope
 //~^ NOTE is `ambig_impl_2_lib::__extensions__::me`

src/test/compile-fail/ambig_impl_unify.rs

@@ -13,11 +13,11 @@ trait foo {
 }
 
 impl foo for ~[uint] {
-    fn foo(&self) -> int {1} //~ NOTE candidate #1 is `__extensions__::foo`
+    fn foo(&self) -> int {1} //~ NOTE candidate #1 is `foo$__extensions__::foo`
 }
 
 impl foo for ~[int] {
-    fn foo(&self) -> int {2} //~ NOTE candidate #2 is `__extensions__::foo`
+    fn foo(&self) -> int {2} //~ NOTE candidate #2 is `foo$__extensions__::foo`
 }
 
 fn main() {