gas: streamline expr_build_dot()

There's no point involving symbol_clone_if_forward_ref(), just for it to
replace dot_symbol by one obtained from symbol_temp_new_now(). For the
abs-section case also produce a slightly more "complete" (as in: all
potentially relevant fields filled) expression by going through
expr_build_uconstant().

Move the function next to current_location(), for it to be easier to see
the (dis)similarities. Correct the function's comment while there.