debug: generate call graph profile data using gprof

This will generate profile data that can be analyzed using gprof. When
you build the application (currently for native_posix only), after
running the application you will get a file "gmon.out" with the call
graph which can be processed with gprof:

  gprof build/zephyr/zephyr.exe gmon.out > analysis.txt

Signed-off-by: Anas Nashif <anas.nashif@intel.com>