sys/arm: add fp[gs]et* prototypes to <ieeefp.h>
We have provided implementations for hard float of these for
a while now. Add them to the header to make things official.
This is required for a bunch of legacy programs in ports.
Approved by: markj (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53156