powerpc: keep FP, VMX and VSX ownership changes atomic with preemptionHEADmain

trap() and set_mcontext() change the FP/VMX/VSX ownership state in
several steps with preemption enabled.  A context switch in between
leaves the thread computing with another thread's register contents:

- enable_fpu()/enable_vec() set PCB_FPU/PCB_VEC before loading the
  registers, so a switch mid-load saves a half-loaded unit over the PCB.
- set_mcontext() clears the frame's MSR bits and then the PCB flags;
  a switch in between re-enables the unit, and the thread returns to
  user space with it enabled but not owned, so it is neither saved nor
  restored until the next signal.

Both paths run after every sigreturn(2), setcontext(2) and
swapcontext(3).  A POWER9 test that keeps f14-f31 live across a signal,
with other threads using the FPU on the same CPU, saw 283 corrupted
round trips out of 882000; none after this change.

Hold a critical section around both, as amd64 and arm64 do.

Reviewed by:	jhibbits
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D60054

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