Fix evaluation order of the cfg_if! macro
Fix evaluation order of the cfg_if! macro

Since #[cfg] attributes are short-circuting, cfg_if! should not change
the order in which the predicates are listed. This enables the use of
unstable cfg attributes in cfg_if!, which is useful when building libstd.

rust-lang/libc,superriva/libc,anatol/libc,cactorium/libc,zethra/libc,anatol/libc,zethra/libc,anatol/libc,mneumann/libc,mneumann/libc,philippkeller/libc,cactorium/libc,rust-lang-nursery/libc,mneumann/libc,philippkeller/libc,rust-lang-nursery/libc,superriva/libc,rust-lang/libc,superriva/libc,zethra/libc,cactorium/libc,rust-lang-nursery/libc,rust-lang/libc,philippkeller/libc