Kernel - Additional cpu bug hardening part 2/2

Kernel - Additional cpu bug hardening part 2/2

* Due to speculative instruction execution, the kernel may
  speculatively execute instructions using data from registers that
  still contain userland-controlled content.

  Reduce the chance of this situation arising by proactively clearing
  all user registers after saving them for syscalls, exceptions, and
  interrupts.  In addition, for system calls, zero-out any
  unrestored registers on-return to avoid leaking kernel data back to
  userland.

* This was discussed over the last few months in various
  OS groups and I've decided to implement it.  After the FP
  debacle, it is prudent to also give general registers similar
  protections.