taskstats: add support for system stats

Add a new command to get system wide information from
userspace. This patch adds system command to export
memory statistics.

Change-Id: I20c1c24f49024f49b21e3d8e88799559fb5b2058
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
[guptap@codeaurora.org: rename nr_indirectly_reclaimable_bytes]
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>
Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>