sjlj exceptions

[Richard Henderson <rth at redhat dot com>]

So, I've got them implemented (largely untested yet), and have realized
that -fsjlj-exceptions really ought to be a multilib if it exists as an
option at all.

It is essentially impossible to mix the two exception models at runtime,
and I'm finding it difficult to manage the two models simultaneously in
libsupc++.  libgcc itself hasn't been overly horrible.

What do folks think about making this choice a configure flag instead of
a runtime flag?  Say --with-sjlj-exceptions to override the default that
gcc would have otherwise chosen. the libraries would have an autoconf
check for what style gcc uses and build accordingly.



r~