kernel - Fix signal FP save/restore issues when AVX is enabled

* The kernel was not saving/restoring the full FP context when entering into
  or returning from a signal, leading to corrupt FP registers even when
  AVX is not used, when AVX is enabled in the kernel.

  ANY SIGNAL COULD CORRUPT THE FP STATE.

* Fixed by adjusting the on-user-stack fpsave area sizes and operation.

* This unfortunately changes a number of user visible structures.
  ucontext_t, mcontext_t, sigcontext, sigframe.

  It is POSSIBLE that most userland use cases will be unaffected, but I'm
  not holding my breath.

Major-Sleuthing-by: ftigeot
Testing-by: ftigeot, dillon
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