duplicated stem loading machinery in cold/warm.lisp

[csziacobus]

warm.lisp essentially duplicates the existing functionality for build-order.lisp-expr. Perhaps a :warm keyword implying :not-host or the like could be implemented as a flag in the latter. This way, we improve maintainability and make it easier to move things from cross compile to warm and vice versa and experiment with bootstrapping PCL machinery in the cross compiler.

[csziacobus (charleszhang77)]

Here's a patch

[Douglas Katzman (dougk)]

Moving PCL into the cross-compiler is a non-goal.

A much simplified change resolves the discrepancy between having two ways to list the sources -
https://sourceforge.net/p/sbcl/sbcl/ci/06dc81c804ffa8b30469f16e88b60c7820f9811e/

Cross-compiling and native compiling are going to invariably need to do a few things differently.
I don't think we should expect to unify the driving scripts completely, so marking this fixed.