rcutorture: Add v3.12 version, which adds sysidle testing

The v3.12 version of the kernel added the CONFIG_NO_HZ_FULL_SYSIDLE
Kconfig parameter, so this commit adds a version transition at that
point.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>