It depends on r323396 r323399 r323440 r323449 r323456 Backporting r323625 should allow lld 6.0 to build a freebsd kernel that works with Xen. If this is too much to backport, I can try to write a more target backport.
For reference I've backported these (and also r323395) into lld 6 in FreeBSD as r328542 through r328548 inclusive. https://svnweb.freebsd.org/changeset/base/328548 I started looking at trying to adapt r323625 to apply directly, but on inspection the dependencies were small and self-contained enough that it made sense to just import the whole series.
I've merged them all in r323733.