Use phase(plugin) in bootstrap crates

Do this to avoid warnings on post-stage0 builds.

src/libsyntax/lib.rs

@@ -32,8 +32,15 @@ This API is completely unstable and subject to change.
 
 extern crate serialize;
 extern crate term;
+
+#[cfg(stage0)]
 #[phase(syntax, link)]
 extern crate log;
+
+#[cfg(not(stage0))]
+#[phase(plugin, link)]
+extern crate log;
+
 extern crate fmt_macros;
 extern crate debug;