Reduce an arm64 VFP critical section

[andrew]

Summary

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

[andrew]

Duplicate of D37998 due to an incorrect git arc command

[andrew]

Abandoned wron review