grab_mcontext() saved the FP and vector registers with save_fpu() and
save_vec(), which drop ownership: PCPU(fputhread) becomes NULL while
PCB_FPU stays set and the thread keeps using the unit. A later
cpu_update_pcb(), at fork(2) or thr_new(2), then skipped the save,
since save_fpu_nodrop() only saves for the thread that owns the unit,
and the child got the registers as they were at the getcontext() or
signal rather than at the fork. Use the nodrop variants.

A child forked after getcontext() saw the parent's f14 as it was at
the getcontext(); it sees the current value now.

MFC after: 1 week