Bump (wip) version  7.5.1 to 7.6
Add patch-opcodes_configure to avoid build error
(at least on NetBSD/amd64 6.1).
---
http://www.gnu.org/software/gdb/news/
April 26th, 2013: GDB 7.6 Released!
    Changes in this release include:
    -   New native configurations (ARM AArch64 GNU/Linux,
        FreeBSD/powerpc, 86_64/Cygwin and Tilera TILE-Gx
        GNU/Linux)
    -   New target configurations (ARM AArch64, ARM AArch64
        GNU/Linux, Lynx 178 PowerPC, x86_64/Cygwin, and Tilera
        TILE-Gx GNU/Linux)
    -   Support for the "mini debuginfo" section, .gnu_debugdata
    -   The C++ ABI now defaults to the GNU v3 ABI
    -   More Python scripting improvements
    -   Some GDB/MI improvements
    -   New configure options, new commands, and options
    -   New remote packets
    -   A new "target record-btrace" has been added while the
        "target record" command has been renamed to "target
        record-full"
    See the NEWS file for a more complete and detailed list of what this release includes.