trace: introduce trace.py prof-lock command

The command shows a profile with amount of time threads were blocked
on a mutex. This can be used to estimate lock contention.

To use the profile the 'mutex_lock_wa*' tracepoints need to be enabled
with backtrace.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
trace: fix symbol resolving for addr2line v2.23

The script did not parse the response properly in case the symbol is
unknown. That version of addr2line responds in two lines:

 ??
 ??:0

but the script was assuming one line ?? ??:0, as returned by addr2line v2.24

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