Add IoctlGetRTCTime and IoctlSetRTCTime to get and set the RTC time via
the RTC_RD_TIME and RTC_SET_TIME ioctls.

Change-Id: I63190026be39746599c2cd7116603d4a1d5bd695
Reviewed-on: https://go-review.googlesource.com/c/160397
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>