kvm tools: Fix compilation on x86_64

Compilation on x86_64 failed with:
[...]
In file included from /usr/include/features.h:386:0,
                 from /usr/include/stdint.h:26,
                 from
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/include/stdint.h:3,
                 from include/kvm/e820.h:4,
                 from bios/e820.c:1:
/usr/include/gnu/stubs.h:7:27: fatal error: gnu/stubs-32.h: No such file
or directory
compilation terminated.
make: *** [bios/bios-rom.bin] Error 1

This is caused by a typo in the Makefile and because gnu/stubs.h (and
friends) expect to see __x86_64__. That's trivial to fix.

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Pekka Enberg <penberg@kernel.org>