acpi: move build_fadt() from i386 specific to generic ACPI source

acpi: move build_fadt() from i386 specific to generic ACPI source

It will be extended and reused by follow up patch for ARM target.

PS:
Since it's generic function now, don't patch FIRMWARE_CTRL, DSDT
fields if they don't point to tables since platform might not
provide them and use X_ variants instead if applicable.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>