printk: Export _vprintk similar to how _prf is exported

There are some corner cases where direct access to the formatter
function is needed. Export _vprintk() so code can use it in a similar
way that _prf() can be directly used.

Change-Id: I9dfb68f87f310e900c662dc8beb320bb4ff7d8b2
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>