Thanks for reporting that.  It appears to be a bug in the garbage collector, and 
is likely to be hard to reproduce.  I couldn't reproduce it, but I did a 'make 
bootstrap' on Fedora x86-64 (configured with --with-wide-int and compiled with 
gcc -m32 so it's really x86), and got a core dump in a completely different area 
that (of course!) went away when I compiled without optimization.

Rather than try to debug it directly, I thought about what might have caused the 
problem, and re-audited the garbage collector with the recent Qnil==0 changes in 
mind.  This did uncover a bug, and the attached patch (which we will need 
anyway) allowed me to do a "make bootstrap" successfully in the same 
configuration.  I installed this into the master as commit 
93f4f67ba93b78e8b31e498e8ce7bce4c8298b76; please give it a try in your setup 
when you have the time.