gas/

    * config/tc-arm.c (md_assemble): Added validation.

    gas/testsuite
    * gas/arm/thumb-w-bad.d: New test case.
    * gas/arm/thumb-w-bad.l: New file.
    * gas/arm/thumb-w-bad.s: New file.
    * gas/arm/thumb-w-good.d: New test case.
    * gas/arm/thumb-w-good.s: New file.