Add antirollback clock support

+ /proc/ar_clock stores the antirollback clock, the earliest time_t
  which the kernel will allow itself to be set to.
+ /proc/ar_clock needs to be set only once, at boot. The kernel will
  continually advance it using the jiffies count.

Ported from:
https://gfiber.googlesource.com/kernel/bruno/+/489304aea9fb0d8d4206c24484bef8f96e0d7e3f%5E%5E!/
https://gfiber.googlesource.com/kernel/bruno/+/6d94cf7eae39171244e1e7171f1df86f6c9c88ef%5E!/

Change-Id: Ibafc784eb4bf491f6a575620d011163b949a6f92
5 files changed