Configuration stripping should happen before checking feature gates

[fhahn]

At the moment, feature_gate::check_crate_macros is executed before strip_unconfigured_items which leads things like #![cfg_attr(stage0, feature(custom_attribute))] in https://github.com/rust-lang/rust/blob/master/src/libcore/lib.rs#L50 beeing ignored (#![feature(custom_attribute)] is again defined in the same file).

The following test illustrates the problem

// compile-flags:--cfg set1
//
#![cfg_attr(set1, feature(custom_attribute))]

#![foobar]
fn main() {}

It fails with The attributefoobaris currently unknown to the the compiler and may have meaning added to it in the future, because of the problem mentioned above.