* config/tc-arm.texi (struct reg_entry): Add new field 'builtin'.

* config/tc-arm.texi (struct reg_entry): Add new field 'builtin'.
  (rn_table, iwmmxt_table, cp_table, cn_table, fn_table, sn_table,
  dn_table, mav_mvf_table, mac_mvd_table, mav_mvfx_table,
  mav_mvax_table, mav_dspc_table): Initialise new field.
  (insert_reg_alias): Initialise new field.
  (md_pseudo_table): Add "unreq" entry.
  (s_unreq): New function: Undo the effects of a previous .req.
* doc/c-arm.texi: Document new pseudo op.
* NEWS: Mention new feature.
* testsuite/gas/arm/req.s: New test file.  Check .req and .unreq psuedo ops.
* testsuite/gas/arm/req.l: Expected error output from req.s test.
* testsuite/gas/arm/copro.d: Set target architecture for objdump so that the
  test will work on architectures which cannot encode higher arm architecture
  types in their file headers.
* testsuite/gas/arm/arm.exp: Run new req.s test.
  Skip thumb instruction test for PE targets which do not support
  thumb relocations.
* testsuite/gas/elf/elf.exp: Skip special handling of section2 test for XScale
  targets - it is no longer needed.