x86: implement user mode on 64-bit

- In early boot, enable the syscall instruction and set up
  necessary MSRs
- Add a hook to update page tables on context switch
- Properly initialize thread based on whether it will
  start in user or supervisor mode
- Add landing function for system calls to execute the
  desired handler
- Implement arch_user_string_nlen()
- Implement logic for dropping a thread down to user mode
- Reserve per-CPU storage space for user and privilege
  elevation stack pointers, necessary for handling syscalls
  when no free registers are available
- Proper handling of gs register considerations when
  transitioning privilege levels

Kernel page table isolation (KPTI) is not yet implemented.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>