Rework makefile to use intermediate variables, not rules.

Rework makefile to use intermediate variables, not rules.

This gives the opportunity for the variables to be files, not phony
rules, to limit rebuilds.  Disadvantage: must be written in
dependency order because make expands variables in dependencies as it
reads them, not lazily.  But this reduces the size of the makefile
quite a bit.

Use stamp_tools to trigger (incremental) rebuild of toolchain.