Reduce an arm64 VFP critical section

In set_fpcontext we only need a critical section around vfp_discard.
The remainder of the code can run without it.

While here add an assert to check the passed in thread is the
current thread as the code already this.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D38000

(cherry picked from commit a85cf421d1bfec7e753bfee781355785b00d89d4)