Tweak slam.sh to be slightly more configurable regarding the host binary
Developers building with an existing SBCL as a host, which is not
installed as "sbcl" in the path, or which needs a --core option, can
now set XC_LISP and XC_CORE prior to invoking slam.sh. Previously,
such a host could only be used with slam.sh after custom
modifications to the script.