gas: isolate macro_strip_at to macro.c

This removes a leftover from i960 support; with that nothing is left
which would set macro_strip_at to non-zero, so the variable is converted
to a #define (retaining the logic in case a new user would appear) and
macro_init()'s respective parameter is dropped.

gas/as.c

@@ -1310,11 +1310,7 @@ gas_init (void)
   expr_begin ();
   eh_begin ();
 
-  int macro_strip_at = 0;
-#ifdef TC_I960
-  macro_strip_at = flag_mri;
-#endif
-  macro_init (flag_macro_alternate, flag_mri, macro_strip_at);
+  macro_init (flag_macro_alternate, flag_mri);
 
   dwarf2_init ();