Improve mlock2 tapset support and add testsuite support.

Improve mlock2 tapset support and add testsuite support.

* tapset/linux/nd_syscalls.stp (nd_syscall.mlock2): Fix argument handling
  and add missing comma between arguments in 'argstr'.
* tapset/linux/syscalls.stp (syscall.mlock2): Add missing comma between
  arguments in 'argstr'.
* testsuite/buildok/aux_syscalls-embedded.stp: Add _stp_mlock2_str()
  compile test.
* testsuite/buildok/nd_syscalls-detailed.stp: Add 'mlock2' compile test.
* testsuite/buildok/syscalls-detailed.stp: Ditto.
* testsuite/systemtap.syscall/mmap.c: Add mlock2 support to test case.