LU-9859 libcfs: merge linux-debug.c into debug.c

LU-9859 libcfs: merge linux-debug.c into debug.c

There is no important difference between the contents
of these files, so merge them into one.

Test-Parameters: trivial
Change-Id: I32ac9215f317b305092623e0743a530e18e4d9c1
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/38602
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>