src: syscall_overhead fix some GCC warnings

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Merge branch 'master' of github.com:netoptimizer/network-testing

Merged needed because I forgot to git pull, before starting to
develop on my laptop.
Resolved merge conflicts

Added a cmpxchg() function in common.h

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
src: add program to time the overhead of cmpxchg

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
src: Generalize timing func from overhead_cmpxchg

Add the generalized timing func from overhead_cmpxchg to common
functions, and use it in syscall_overhead.c

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>