Improve gas syntax .set directive to better match GNU as behavior.

- Add support for deferred symbols (those which depend on other symbols
  that have not yet been .set)
- Add support for re-defining symbols
- Add support for 0x hex notation in literals

Contributed by: Alexei Svitkine <alexei.svitkine@gmail.com>

svn path=/trunk/yasm/; revision=2366