Mingw-w64 exception handling configuration

[Sandra Loosemore]

I see that the default EH behavior for a biarch mingw-w64 target GCC is 
to use SJLJ for the 32-bit multilib and SEH for the 64-bit one, but that 
there are #errors in cygming.h and mingw32.h that prevent you from 
configuring a biarch build with --disable-sjlj-exceptions to use DWARF-2 
and SEH, respectively.  Is there a deep reason for this, or is this just 
a case where nobody updated the configuration logic after SEH was 
implemented?  I understand that before that, you had to use SJLJ for 
64-bit targets because the DWARF-2 support is restricted to 32-bit 
targets, so it was completely reasonable to error out.

The reason I'm asking is, I have a third-party 32-bit library that was 
built for DWARF-2 EH....

-Sandra