Rejigger pathnames and such to facilitate sandboxed builds.
These changes are among those necessary to build SBCL in a tree that provides
all and only files that are declared requisites to each step,
whose contents are nothing but symlinks into an abstract repository.
- Don't use TRUENAME. It doesn't correlate in any way to a real name.
All parts of the apparent "true" name are meaningless identifiers.
- Delay reading "build-order.lisp-expr" except when required
by use of the DO-STEMS-AND-FLAGS macro, and not before.
- Do not read parameter files "dynamic-space-size.txt" and "version.lisp-expr"
unless building the cross-compiler.