scripts/gdb: add lx-dmesg command

This pokes into the log buffer of the debugged kernel, dumping it to the
gdb console.  Helping in case the target should or can no longer execute
dmesg itself.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Borislav Petkov <bp@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>