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: re (cperciva)
Approved by: markj (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53156

(cherry picked from commit a8079d40ae7f3cee17c94e61e43c24780a64a010)
(cherry picked from commit af39e511ee158edc469f2f73aaa5bc5af872c747)