printk: add support for lockless ringbuffer

Linux 5.10 moved to a new lockless ringbuffer. The new ringbuffer
is structured completely different to the previous iterations.
Add support for retrieving the ringbuffer using vmcoreinfo. The
new ringbuffer is detected based on the availability of the
"prb" symbol.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Simon Horman <horms@verge.net.au>