linuxkpi: #ifdef guard fpu_kern(9) usage

The recent revert of 8ca78eb03fd4 removed the guards around these calls.
As a result, LINT builds for architectures which do not implement this
API on the branch are failing -- armv7, powerpc, powerpc64, riscv.

This is a direct commit to stable/13.

Fixes: fa6603d4a725 (Revert "LinuxKPI: Make FPU sections thread-safe...")
Differential Revision: https://reviews.freebsd.org/D40497