Remove function pointers in output, simplify error handling

Remove function pointers in output, simplify error handling

Remove a bunch of function pointers in the output stage; they are
never changed and don't add any value.  Also make "ofile" a global
variable and let the backend use it directly.

All we ever did with these variables were stashing it in locals and
using them as-is anyway for no benefit.

Also change the global error function, nasm_error() into a true
function which invokes a function pointer internally.  That lets us
use direct calls to it.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>