trace: use monotonic clock for trace samples

Tracing tools assume that timestamps are monotonic. Trace buffer
merging relies on that when it produces a stream sorted by timestamps.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
trace: adjust unimportant_prefixes

The prefix changed probably after arch-trace.hh was introduced.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>