Add support for --gc-sections

2013-01-07  Simon Cook  <simon.cook@embecosm.com>

       * bfd/elf32-arc.c: elf_backend_can_gc_sections.
       * ld/testsuite/ld-selective/sel-dump.exp: Remove arc from xfail.
       * ld/testsuite/ld-selective/selective.exp: Likewise.