Add support for the ARM speculation barrier instructions SSBB and PSSBB.

opcodes * arm-dis.c (arm_opcodes): Add ssbb and pssbb and move
     csdb together with them.
     (thumb32_opcodes): Likewise.

gas  * config/tc-arm.c (insns): Add new ssbb and pssbb instructions.
     * testsuite/gas/arm/csdb.s: Add new tests for ssbb and pssbb.
     * testsuite/gas/arm/csdb.d: Likewise
     * testsuite/gas/arm/thumb2_it_bad.s: Likewise.
     * testsuite/gas/arm/thumb2_it_bad.l: Likewise.
     * testsuite/gas/arm/barrier.d: Update with ssbb.
     * testsuite/gas/arm/barrier-thumb.d: Likewise.