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).

(Originally commited in a015186f)
(cherry-pick of commit: 3458ec13)

Bug: 37916906
Bug: 36574794
Bug: 62101480
Test: Builds and boots.
Change-Id: I83aa392f49bb412d96534925fb02921a8f4731fa