Using __sync_* builtins within libgcc code

[Luke Dalessandro]

I'm making some modifications to exception handling inside of unwind-dw2-fde.c 
that I'd like to use __sync_bool_compare_and_swap for, unfortunately I can't 
seem to figure out how to correctly use builtins in the context of libgcc.

I've tried a bunch of different things, but I consistently get the error

   bin/../lib/gcc/i686-pc-linux-gnu/4.3.1/../../../libgcc_s.so: undefined 
reference to `__sync_bool_compare_and_swap_4'

so clearly I'm not getting this symbol included correctly when I build. Any 
advice would be appreciated.

Luke