* as.c (main): Mark symbols created via the --defsym command line

* as.c (main): Mark symbols created via the --defsym command line
  option as volatile so that they can be overridden later on by a
  .set directive.  This maintains compatibility with the behaviour
  of earlier versions of the assembler.
* doc/as.texinfo (--defsym): Document that the defined symbol's
  value can be overridden via a .set directive.