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).

(Originally commited in a015186f)
Bug: 36574794
Bug: 62101480
Test: Builds and boots.
Change-Id: I249e11291c58fee77098dec3fd3271ea23363ac9