Use the correct length when copying arm64 vfp registers

We passed the wrong length into memcpy in the arm64 get_fpcontext and
set_fpcontext. This caused us to copy two status registers we didn't
expect to copy.

These are safe as they exist in both the source and destination, although
in a different order, and we copy the correct values after the memcpy.

Obtained from: CheriBSD
Sponsored by: Innovate UK