A completely different approach to EH runtime

[Joe Buck]

HJ writes:
> No. I don't think we should do that. I am talking about adding new
> symbols. In that case, you have to make sure
> 
> 1. The new libgcc is 100% compatible with the old one.

Yes, I think that we all agree that this is a requirement: any new libgcc
must provide all of the old symbols, with 100% compatible ABIs.

> 2. Only the new libgcc is visible to ld.so.

Because dynamic linking may have subtle differences on different systems,
we will perhaps need OS-specific instructions to make sure that this is
the case.  On SVR4-like systems, having the new libgcc be the first match
in LD_LIBRARY_PATH should suffice.