Fix sigevent on DragonFly.
Fix sigevent on DragonFly.

The union which contains a void* has to be aligned at a 8-byte boundary.

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