benchtests: Add pthread-mutex-locks bench

Benchmark for testing pthread mutex locks performance with different
threads and critical sections.

The test configuration consists of 3 parts:
1. thread number
2. critical-section length
3. non-critical-section length

Thread number starts from 1 and increased by 2x until num of CPU cores
(nprocs). An additional over-saturation case (1.25 * nprocs) is also
included.
Critical-section is represented by a loop of shared do_filler(),
length can be determined by the loop iters.
Non-critical-section is similiar to the critical-section, except it's
based on non-shared do_filler().

Currently, adaptive pthread_mutex lock is tested.