FreeBSD: Don't add -L/usr/local/lib{,/gcc4[46]}.

The first one in particular results in Rust not being able to build itself
if it is installed.  The latter two shouldn't be necessary, and should only
be included if they are actually going to be used.

src/librustc_back/target/freebsd_base.rs

@@ -18,11 +18,6 @@ pub fn opts() -> TargetOptions {
         executables: true,
         morestack: true,
         has_rpath: true,
-        pre_link_args: vec!(
-            "-L/usr/local/lib".to_string(),
-            "-L/usr/local/lib/gcc46".to_string(),
-            "-L/usr/local/lib/gcc44".to_string(),
-        ),
 
         .. Default::default()
     }