Replace walk_stack field version guard with autoconf test.

Replace walk_stack field version guard with autoconf test.

The test for whether or not to assign print_context_stack to the
walk_stack stacktrace_ops field depended on the kernel version.
Replace with a proper runtime/autoconf test to make sure the field
always gets assigned when available.

* buildrun.cxx (compile_pass): Add output for STAP_CONF_WALK_STACK.
* runtime/autoconf-walk-stack.c: New test.
* runtime/stack.c (print_stack_ops): Assign walk_stack field
  print_context_stack depending on STAP_CONF_WALK_STACK.