Do build-time bindgen

Majority of build_gecko.rs is just the straightforward conversion from
regen.py. There are two differences that:
1. Side in whitelist is changed to mozilla::Side
2. std::atomic__My_base is added to opaque types for Windows

ports/geckolib/Cargo.toml

@@ -9,6 +9,9 @@ name = "geckoservo"
 path = "lib.rs"
 crate-type = ["staticlib", "rlib"]
 
+[features]
+bindgen = ["style/bindgen"]
+
 [dependencies]
 app_units = "0.3"
 cssparser = {version = "0.7"}