Add missing ELFObjectWriter::reset() override, like other MC classes.

See detailed discussion at

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140915/235418.html

and r217907, r217948:

 http://llvm.org/viewvc/llvm-project?view=revision&revision=217907
 http://llvm.org/viewvc/llvm-project?view=revision&revision=217948

llvm-svn: 232982