ptxed: track number of blocks

When using ptxed's --stat option with the block decoder, print the number of
blocks in addition to the number of instructions.

Also add a new option --stat:blocks to select collecting the number of blocks.

Change-Id: I59caf3692667318f23a1887a844042b561a338a5
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>