PR 9871 (partial) fix. Removed some embedded-C in context.stp and task.stp.

PR 9871 (partial) fix.  Removed some embedded-C in context.stp and task.stp.

* tapset/context.stp: Converted several functions from embedded-C to
  script language.
* tapset/task.stp (task_parent): Converted from embedded-C to script
  languange.
* testsuite/buildok/context-embedded.stp: Moved function tests not
  actually present in context.stp to their own files.
* testsuite/buildok/context-symbols-embedded.stp: New testcase (includes
  some tests from context-embedded.stp).
* testsuite/buildok/context-unwind-embedded.stp: Ditto.
* testsuite/buildok/ucontext-symbols-embedded.stp: Ditto.
* testsuite/buildok/ucontext-unwind-embedded.stp: Ditto.
* testsuite/systemtap.pass1-4/buildok.exp: Kfails the ucontext tests if
  the system doesn't support uprobes.