1) Group front end objects in Makefile.in under e.g. ALL_HOST_FRONTEND_OBJS
2) Add a new build rule that adds an extra define -DIN_GCC_FRONTEND
3) Conditionally poison symbols in system.h
For the last step, that would be e.g.:
#ifdef IN_GCC_FRONTEND
#pragma GCC poison GCC_RTL_H GCC_REGS_H ...
#endif
Unfortunately, my Makefile-fu is insufficient to do step 2. There is
an implicit rule ".c.o:" that triggers no matter what I try, and I
don't want to add explicit $(COMPILER) rules for all the front end
objects.