Set the 'FR' bit in the status register for N32 kernels.

This permits N32 hard-float binaries to use 64-bit floating point
registers (which is what N32 binaries expect) matching the N64 ABI.

Reviewed by: imp, jmallett
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D13830