Cleaning up LINKER_FEATURES ifunc and build-id

From: Ed Maste <emaste_at_freebsd.org>
Date: Tue, 02 Jun 2026 18:17:49 UTC
All linkers supported by our build support ifunc and build-id, and
these LINKER_FEATURES are no longer tested in the tree. I plan to
remove them from bsd.linker.mk shortly -- see
https://reviews.freebsd.org/D55677