aarch64: Fix get_fpcontext32() to work on non-curthread.

Similar to fill_fpregs(), only invoke vfp_save_state() for curthread.

While here, zero the buffer if FP hasn't been started to avoid leaking
kernel stack memory.

Reviewed by: andrew, markj
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34525