powerpc: cpuset: add local functions for copyin/copyout

Add local functions to workaround an instruction segment trap (panic)
when the indirect functions copyin and copyout are called by an external
loadable kernel module (i.e. pfsync, zfs and linuxulator). The crash
was triggered by change 47a57144af25a7bd768b29272d50a36fdf2874ba, but
kernel binary linked with LLD 9 works fine. LLVM bisect points that LLD
behavior chaged after dc06b0bc9ad055d06535462d91bfc2a744b2f589.

This is know to affect powerpc targets only and the final fix is still
being discussed with the LLVM community.

PR: 266730
Reviewed by: luporl, jhibbits (on IRC, previous version)
MFC after: 2 days
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D36234