Clarify that include/opcode/ files are part of GNU opcodes

include/ChangeLog:
2017-01-25  Dimitar Dimitrov  <dimitar@dinux.eu>

        * opcode/hppa.h: Clarify that file is part of GNU opcodes.
        * opcode/i860.h: Ditto.
        * opcode/nios2.h: Ditto.
        * opcode/nios2r1.h: Ditto.
        * opcode/nios2r2.h: Ditto.
        * opcode/pru.h: Ditto.

opcodes/ChangeLog:
2017-01-25  Dimitar Dimitrov  <dimitar@dinux.eu>

        * pru-opc.c: Remove vague reference to a future GDB port.