kvm tools: Fix E820 map on x86-64

We must not define '-D__x86_64__' for all the code because the BIOS emulation
code is actually real mode 16-bite code. This fixes E820 map regression on
x86-64 caused by commit e227ea5 ("kvm tools: Fix compilation on x86_64").

Signed-off-by: Pekka Enberg <penberg@kernel.org>