xen/x86: don't use '.ifnes' in bug frame construction.

xen/x86: don't use '.ifnes' in bug frame construction.

Spotted because it breaks the clang build for LLVM <3.2.  .ifnes is
not right here as it will choke on a string with embedded quotes.

.ifnb would be better except that LLVM <3.2 doesn't support that either
(and nor does binutils 2.16).

It should be possible to use something like !!msg or !!msg[0] instead
of a separate flag, but I gave up trying to find something that would
make it through CPP, asm() and gas as a usable constant. :|

Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>