libprocstat: extend zfs_defs hack for .pieo

By default _pie.a archives are built only for INTERNALLIBs, so there is
usually no need for zfs_defs.pieo to exist. However, some experimental
work builds _pie.a archives for everything. Extend the existing set of
zfs_defs hacks to build zfs_defs.pieo as well.

Reviewed by: arichardson
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31924