Add /dev/kmsg_debug.

Add /dev/kmsg_debug on userdebug devices, to allow crash_dump to log
crashes to dmesg when logd isn't up yet (or is the one crashing).

Bug: http://b/36574794
Test: stop tombstoned; crasher; dmesg
Change-Id: I249e11291c58fee77098dec3fd3271ea23363ac9