Read constants from the RTS unit
Read constants from
DerivedConstants.h in the RTS unit instead of from the globally installed
platformConstants (no longer produced after this patch). It allows us to have a single source of truth (the header file) used for C/Cmm/Hs codes and more importantly to start supporting more than one target platform.
It is particularly useful for #14335 which needs a different
Platform for the host for plugins and
Platform needs platform constants.