Add support for CPUID PREFETCHWT1

Add support for CPUID PREFETCHWT1

Latest AVX512 spec
http://download-software.intel.com/sites/default/files/managed/50/1a/319433-018.pdf
Has CPUID PREFETCHWT1 for prefetchwt1 instruction, which we list as AVX512PF.
This patch introduces CPUID PREFETCHWT1.

gas/

        * config/tc-i386.c (cpu_arch): Add .prefetchwt1.
        * doc/c-i386.texi: Document .prefetchwt1/prefetchwt1.

opcodes/

        * i386-gen.c (cpu_flag_init): Add CPU_PREFETCHWT1_FLAGS/
        (cpu_flags): Add CpuPREFETCHWT1.
        * i386-init.h: Regenerate.
        * i386-opc.h (CpuPREFETCHWT1): New.
        (i386_cpu_flags): Add cpuprefetchwt1.
        * i386-opc.tbl: Cahnge CPU of prefetchwt1 from CpuAVX512PF to CpuPREFETCHWT1.
        * i386-tbl.h: Regenerate.

gas/testsuite

        * gas/i386/avx512pf-intel.d: Remove prefetchwt1.
        * gas/i386/avx512pf.s: Ditto.
        * gas/i386/avx512pf.d: Ditto.
        * gas/i386/x86-64-avx512pf-intel.d: Ditto.
        * gas/i386/x86-64-avx512pf.s: Ditto.
        * gas/i386/x86-64-avx512pf.d: Ditto.
        * gas/i386/prefetchwt1-intel.d: New file.
        * gas/i386/prefetchwt1.s: Ditto.
        * gas/i386/prefetchwt1.d: Ditto.
        * gas/i386/x86-64-prefetchwt1-intel.d: Ditto.
        * gas/i386/x86-64-prefetchwt1.s: Ditto.
        * gas/i386/x86-64-prefetchwt1.d: Ditto.