Refine defininition of mcontext_t on x86-x64.
Refine defininition of mcontext_t on x86-x64.

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