Fix bug: incorrect callsite stack traceback during .so file handling

[sb17v]

Read the offset from /proc/<pid #>/maps and add it to PC to handle callsite stack traceback correctly for .so files

[artpol84]

@cchambreau we were trying to profile some MPI application and noticed that the call stack is broken. IMO it’s one of the greatest features in mpiP so we thought it should be fixed

[artpol84]

@sb17v patch looks clean, thanks!