s390/debug: make debug event time stamps relative to the boot TOD clock

The debug features currently uses absolute TOD time stamps for the
debug events. Given that the TOD clock can jump forward and backward
due to STP sync checks the order of debug events can get obfuscated.

Replace the absolute TOD time stamps with a delta to the IPL time
stamp. On a STP sync check the TOD clock correction is added to
the IPL time stamp as well to make the deltas unaffected by STP
sync check.

The readout of the debug feature entries will convert the deltas
back to absolute time stamps based on the Unix epoch.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>