Lookup the start,len of the "execve" stub in libc.so, and tell the kernel

Lookup the start,len of the "execve" stub in libc.so, and tell the kernel
where it is with pinsyscall().  In non-static binaries, kernel execve(2)
will now insist upon being called from that small region.  Arriving from
a different region smells like ROP methodology, and the process is killed.