gdb/testsuite/gdb.dwarf2: Replace @ with % for ARM compatability.

The ARM assembler recognises @ symbol as a comment marker and
therefore is unable to assemble some of gdb.dwarf2 testsuite tests.
This patch replaces all occurrences of @ symbol with % sign to allow
tests stay compatible with both ARM and x86 assembly. This patch has
been tested on x86_64 and ARMv7 targets.

gdb/testsuite/ChangeLog:

2013-07-19  Omair Javaid  <Omair.Javaid@linaro.org>

        * gdb.dwarf2/fission-reread.S: Replace @ sign with % sign to remove
        assembler errors on arm.
        * gdb.dwarf2/mac-fileno.S: Likewise.
        * gdb.dwarf2/member-ptr-forwardref.S: Likewise.
        * gdb.dwarf2/pr13961.S: Likewise.