tools/gdbsx: fix build with clang 3.8

tools/gdbsx: fix build with clang 3.8

Currently building gdbsx with clang 3.8 gives the following errors:

xg_main.c:783:17: error: 'aligned' attribute ignored when parsing type [-Werror,-Wignored-attributes]
    iop->uva = (uint64_aligned_t)((unsigned long)tobuf);
                ^~~~~~~~~~~~~~~~
/builds/liuw/xen/tools/debugger/gdbsx/xg/../../../../tools/include/xen/arch-x86/xen-x86_32.h:105:50: note: expanded from macro 'uint64_aligned_t'
                                                 ^~~~~~~~~~
xg_main.c:816:17: error: 'aligned' attribute ignored when parsing type [-Werror,-Wignored-attributes]
    iop->uva = (uint64_aligned_t)((unsigned long)frombuf);
                ^~~~~~~~~~~~~~~~
/builds/liuw/xen/tools/debugger/gdbsx/xg/../../../../tools/include/xen/arch-x86/xen-x86_32.h:105:50: note: expanded from macro 'uint64_aligned_t'

According to https://bugs.llvm.org/show_bug.cgi?id=11071, this issue has
been fixed in clang. But we're not going to get that in 3.8.

Explicitly disable that warning to fix the build.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>