Patch: Imported boehm-gc 5.0alpha6

[Tom Tromey]

Bryce> Anyone know what the status of GC_gcj_fast_malloc() is? Should
Bryce> we be using it?

Tom> We have a flag in the compiler, -fuse-boehm-gc, that generates
Tom> the tables that this code in the GC expects.  However, I have
Tom> never tried hooking the two together.

Oh, yeah -- this work is supposed to speed up marking.
So if you want to play with this it would be useful to get
before/after benchmark numbers for something that exercises the GC.

Tom