Remove btxld from symlinked host tools

It is only present on amd64/i386 systems which breaks buildworld on
other hosts. In fact there is no need to add it to the bootstrap tools
list since it is already included in the cross-tools phase.
However, for cross-tools it was only built if the host and target
architecture didn't match. After this change it is also built when we
are builtin with a strict $PATH.

Reported By: mmel