src.libnames.mk: Add hack to workaround libarchive not being bootstrapped

We don't currently honour _DP_foo when bootstrapping on non-FreeBSD, and
so none of these values matter, but the next commit will change that as
we do need to pull in dependencies for libdwarf. We should really be
bootstrapping our libarchive for ar anyway rather than using the host's,
as well as have a better way to communicate to src.libnames.mk whether
or not a library is being bootstrapped.

MFC after: 1 week