build-sys: Update warning message
build-sys: Update warning message

arch_warn was set to 1 only if architecture is not x86, x64 or arm.
Update the message as we actually mainly test x64.
Define the warning message and do the architecture checks in the
same place so that they are easier to keep in sync.

Signed-off-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>
Acked-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>

ya-mouse/spice,SPICE/spice,fgouget/spice,fgouget/spice,SPICE/spice,fgouget/spice,ya-mouse/spice,fgouget/spice,SPICE/spice,ya-mouse/spice