build system: avoid warning about missing .xcompile

That file will be generated, but not before make managed
to complain about it.
So let's just generate it if missing - it won't hurt the
dependency tracking some lines later which is looking at
time stamps.

Change-Id: I615f38457eb27a8ffb4352b5234e262ee95d84ac
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6305
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>