Changes:
- Isabelle2017 compat for output heaps is now removed.
- We no longer select x86_64 as the ML_PLATFORM; the new default
x86_64_32 platform is generally superior.
- We still select ML_PLATFORM for Isabelle2018 compat.
ML_HOME is not always set already when this script is sourced (e.g. when
the polyml component is somehow faulty).
Isabelle heap output settings are now part of the TS isabelle patch queue;
explicit override is still possible as before.
Japheth Lim
Edward Pierzchalski
Gerwin Klein
Alejandro Gomez-Londono
Daniel Matichuk
Gerwin Klein