The main problem was bitrot after elftoolchain being swapped in for the
GNU toolchain.
This also reworks how the list of 'host allowed' libraries is determined
to only allow INTERNALLIBs, which is needed for libelftc to come in.
For usr.bin/readelf use the same hack, as libelf and libdward, to bring in
the needed sys/ headers for host builds. This has not yet been a problem due
to readelf not being built as a host tool in buildworld. This is possible
in the meta build though when building the toolchain.
Sponsored by: EMC / Isilon Storage Division
