smbios: build legacy mode code only for 'pc' machine

smbios: build legacy mode code only for 'pc' machine

basically moving code around without functional change.
And exposing some symbols so that they could be shared
between smbbios.c and new smbios_legacy.c

plus some meson magic to build smbios_legacy.c only
for 'pc' machine and otherwise replace it with stub
if not selected.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Message-Id: <20240314152302.2324164-12-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>