Add new 'z' format for print command.

Add new 'z' format for print command.

  http://sourceware.org/ml/gdb-patches/2013-07/msg00235.html

gdb/ChangeLog

        * NEWS: Mention new 'z' formatter.
        * printcmd.c (print_scalar_formatted): Add new 'z' formatter.
        (_initialize_printcmd): Mention 'z' formatter in help text of the
        'x' command.

gdb/doc/ChangeLog

        * gdb.texinfo (Output Formats): Mention the new 'z' formatter.

gdb/testsuite/ChangeLog

        * gdb.base/printcmds.exp (test_print_int_arrays): Add tests for x,
        z, o, and t output formats.
        * gdb.base/display.exp: Use 'k' as an undefined format now that
        'z' is defined.