Imported Boehm GC 5.0 release

[Tom Tromey]

Bryce> I've imported the 5.0 release of the collector. Again, the
Bryce> changes are minor, but now we can say we're not running an
Bryce> "alpha" release!

Thanks Bryce; much appreciated.

Probably one more import at some point in the future: Hans might add
finalizer notification to the GC so that we can know when to run
finalizers (in a separate thread).

Tom