Diagnostic advice for "Internal compiler error in ?" ?

I've got a thorny problem, and I could use some advice about how to
diagnose it.  I've got an ancient version of GCC (3.2.3), built for a
platform GCC hasn't supported in years (IBM System/370), and using a
back-end that was removed from GCC 20 years ago (i370).  Upgrading isn't an
option - this is the newest there is.  Switching to another compiler isn't
either - this is all there is.

The error message is literally:

rxvmvars.h: In function `extract_double_decimal':
rxvmvars.h:1547: Internal compiler error in ?, at <stdin>:902
Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:http://gcc.gnu.org/bugs.html> for instructions.

I'm not asking anyone to chase this bug, but other that just randomly
cutting code out of my program over and over until it goes away, are there
hints I can follow for this?  I'm a very experienced OS-level programmer,
so I've got the background to dig into this.  But "Internal compiler error
in ?" isn't much of a pointer to follow :-)

Thanks,
Ross Patterson