Only mess with VFP state on the CPU for curthread for get/set_vfpcontext.

Future changes will use these functions to fetch and store VFP state for
threads other than curthread.

Reviewed by: andrew, stevek, Michal Meloun <meloun-miracle-cz>
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D12292