implement #[nolink]; deprecate #[link_name = ""]; note in stdlib to remove empty link_name.

Can't remove them from stdlib until the snapshotted compiler supports #[nolink].

src/libstd/linux_os.rs

@@ -24,7 +24,8 @@ export fsync_fd;
 // FIXME Somehow merge stuff duplicated here and macosx_os.rs. Made difficult
 // by https://github.com/graydon/rust/issues#issue/268
 
-#[link_name = ""]
+#[link_name = ""]               // FIXME remove after #[nolink] is snapshotted
+#[nolink]
 #[abi = "cdecl"]
 native mod libc {
     fn read(fd: fd_t, buf: *u8, count: size_t) -> ssize_t;