kernel - Fix floating point save state structure and minor npx issues

kernel - Fix floating point save state structure and minor npx issues

* The floating point save structure(s) used by the kernel and possibly
  also userland were too large for x86-64 due to a porting error where
  'long' variables were left intact that should have been turned into
  32-bit variables.

  No known adverse effect to the too-large structures but we have to get
  it right.

* npxexit() was not being called in a kernel thread exit case.  Kernel
  threads do not use the FP unit so the case was never hit, but fix it
  anyway.

* Move a critical section to cover a flags test to handle a very rare
  preemptive thread switch issue.  Since the preempting thread is a
  kernel thread which does not use the FP unit this case was never hit,
  but fix it anyway.