java/builtins.c needs to call can_compare_and_swap, which happens to be 
provided via optabs.h.
As a result, we see the following in java/builtins.c:

/* FIXME: All these headers are necessary for sync_compare_and_swap.
    Front ends should never have to look at that.  */
#include "rtl.h"
#include "insn-codes.h"
#include "expr.h"
#include "optabs.h"

by providing a target hook and a default which simply calls 
can_compare_and_swap_p(),  we can remove all that include crap from the 
front end.  None of those 4 include files are now directly included by 
any front end files.

Bootstraps on x86_64-unknown-linux-gnu, and no new testsuite failures.  
OK for trunk?

Andrew