rustbuild: Sync some Cargo.toml/lib.rs dependencies

The standard library doesn't depend on rustc_bitflags, so move it to explicit
dependencies on all other crates. Additionally, the arena/fmt_macros deps could
be dropped from libsyntax.

src/libsyntax/Cargo.toml

@@ -9,8 +9,7 @@ path = "lib.rs"
 crate-type = ["dylib"]
 
 [dependencies]
-arena = { path = "../libarena" }
-fmt_macros = { path = "../libfmt_macros" }
 serialize = { path = "../libserialize" }
 term = { path = "../libterm" }
 log = { path = "../liblog" }
+rustc_bitflags = { path = "../librustc_bitflags" }