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?

We have a flag in the compiler, -fuse-boehm-gc, that generates the
tables that this code in the GC expects.
However, I have never tried hooking the two together.
I didn't know that Hans put this code into the released GC -- cool
stuff.  We should definitely give it a try.

Tom