crtstuff computes __CTOR_END__ - 1, and this ICEd while testing
-fcompare-debug.  It's normally compiled with -g0, but I introduced
-gtoggle for -fcompare-debug, so it ended up compiled for comparison
with -g2, and the assignment p = __CTOR_END__ - 1; generated a debug
stmt with the expression &__CTOR_END__[-1], which exposed the failure
of expand_debug_expr() to handle negative bitpos.  Fixed as follows.