Patch: replace mutex with object synchronization

[Tom Tromey]

Bryce> I guess it doesn't matter right now, since libgcj itself isn't
Bryce> built with the BC-ABI, but eventually, using Java locks here
Bryce> could potentially lead to circularity problems.

I suspect there will always be a few classes we don't compile with the
BC ABI.  Object, maybe Class and String too.  This seems like an area
we need to experiment with once the patches are in.

Tom