Invalidate cache before first jump

Some CPUs (Sandybridge) seem to require this, and it does not hurt
on other CPUs.

Change-Id: I4fdb281b2b684ab5fea999aae28ca08dce24da4d
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/869
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)