csu: Fix dynamiclib/init_test:jcr_test on !HAVE_CTORS archs

.jcr still needs a 0-entry added in crtend, even on !HAVE_CTORS archs, as
we're still getting .jcr sections added -- presumably due to the reference
in crtbegin. Without this terminal, the .jcr section (without data) overlaps
with the next section and register_classes in crtbegin will be examining the
wrong item.

PR: 241439
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D22132