Specify -z notext when building with -z ifunc-noplt.

The upstream implementation of -z ifunc-noplt disallows its combination
with -z text. The option does not have much significance for kernel
builds, though.

Reviewed by: kib (previous version)
Discussed with: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20260