Implement sigsetjmp and siglongjmp for X64
This change implements sigsetjmp and siglongjmp functions for X64, based on their Musl implementation. The change also enlarges jmp_buf to 16 unsigned longs. The change exports the setjmp and longjmp functions from the setjmp header because although they were exported for C, but other Rust headers couldn't reference it. Tests are included for sigsetjmp and siglongjmp, executed successfully both on Linux and Redox.