kernel - Add syscall quick return path for x86-64

kernel - Add syscall quick return path for x86-64

* Flag the case where a sysretq can be performed to quickly return
  from a system call instead of having to execute the slower doreti
  code.

* This about halves syscall times for simple system calls such as
  getuid(), and reduces longer syscalls by ~80ns or so on a fast
  3.4GHz SandyBridge, but does not seem to really effect performance
  a whole lot.

Taken-From: FreeBSD (loosely)