* config/tc-arm.c (BAD_ADDR_MODE): Define.

* config/tc-arm.c (BAD_ADDR_MODE): Define.
  (arm_reg_parse_multi): Return NULL rather than FAIL.
  (arm_reg_parse): Fix comment, the function returns FAIL rather than NULL if
    it is unable to parse the register name.
  (do_ldrex): Use BAD_ADDR_MODE.
    Change error message for PC-relative addressing.
  (do_strex): Likewise.
  (do_t_ldrex): Use BAD_ADDR_MODE.
  (do_t_strex): Likewise.
* gas/arm/archv6t2-bad.s: Add tests of badly composed ldrex and	strex
    instructions.
* gas/arm/archv6t2-bad.l: Add expected error messages.
* gas/arm/r15-bad.l: Adjust error messages for r15 usage in ldrex and strex
    instructions.