IBM Z: Implement instruction set extensions

opcodes/

        * s390-mkopc.c (main): Accept arch14 as cpu string.
        * s390-opc.txt: Add new arch14 instructions.

include/

        * opcode/s390.h (enum s390_opcode_cpu_val): Add
        S390_OPCODE_ARCH14.

gas/

        * config/tc-s390.c (s390_parse_cpu): New entry for arch14.
        * doc/c-s390.texi: Document arch14 march option.
        * testsuite/gas/s390/s390.exp: Run the arch14 related tests.
        * testsuite/gas/s390/zarch-arch14.d: New test.
        * testsuite/gas/s390/zarch-arch14.s: New test.