vkernel - Fix FP corruption from preemptive thread switch

vkernel - Fix FP corruption from preemptive thread switch

* Recent work on the vkernel enabled preemptive interrupt thread switching.
  This introduced a race where the floating point state could change
  out from under the vkernel's go_user() function, causing the FP state
  in the virtual user process to become corrupt.

* Fixed by introducing a critical section which essentially defers any
  preemptive thread switches inside go_user().

Reported-by: YONETANI Tomokazu <qhwt+dfly@les.ath.cx>
Test-cases-by: YONETANI Tomokazu <qhwt+dfly@les.ath.cx>