MT safe exception diffs

[Jason Merrill]

Thanks.  A few comments:

The thread handling stuff needs to be independent of the EH code.  The EH
code should use a single interface to the various threads packages, and
the Objective-C frontend should use the same interface.  The current objc
thread code will probably be useful as a starting point.

eh_context::top_elt doesn't need to be thread-specific, as it is only used
as a null value.  For that matter, I don't know why we don't just use 0.
Mike?

eh_context::buf should be #ifndef DWARF2_UNWIND_INFO.

It would be nice, though certainly not necessary, to get support for nested
exceptions at the same time as thread safety.  The upshot will be that
instead of information for a single active exception you have a stack of them.

Jason