While I was looking at the interaction of delayed folding with GGC, I 
noticed that ggc_handle_finalizers currently runs no finalizers if 
G.context_depth != 0.  So any GC objects in a greater depth will still 
be collected, but they won't have their finalizers run.  This 
specifically affects compiles that use a PCH file, since G.context_depth 
is set to 1 after loading the PCH.

This patch fixes ggc_handle_finalizers to look at the depth of each 
finalizer so that we still don't try to run finalizers for 
non-collectable objects loaded from the PCH, but we do run finalizers 
for collectable objects allocated after loading the PCH.

I ended up not relying on this for delayed folding, but it still seems 
like a good bug fix.

Tested x86_64-pc-linux-gnu.  OK for trunk?