run-sbcl.sh set BASE using realpath and dirname

If SBCL_HOME is a relative path then it is computed relative
to *default-pathname-defaults*. When running asdf:load-system
*default-pathname-defaults* is set to the path of the system
being loaded. If there is a call to (require 'thing) anywhere
in the system definition then SBCL_HOME will be expanded
incorrectly and the require will fail.

Since SBCL_HOME is set from BASE in run-sbcl.sh this patch is
the simplest fix to restore correct behavior. I do not know
whether SBCL_HOME should ever be allowed to be a relative path
and it doesn't seem that ASDF is doing anything wrong here
since the issue is in the package that is calling require at
a somewhat strange time.