Many small tapset/syscalls2.stp fixups. Added testcase.

Many small tapset/syscalls2.stp fixups.  Added testcase.

* tapset/syscalls2.stp (syscall.compat_sys_recvmsg): Make sure it supports
  the same variables as the non-compat probe.
  (syscall.compat_sys_semctl): Ditto.
  (syscall.compat_sys_sendmsg): Ditto.
  (syscall.sched_setaffinity): At some point, x86_64 kernels couldn't
  find $len, so a special version of this probe was written for it.  Now
  just uses @defined().
  (syscall.compat_signalfd): Prefers compat_sys_signalfd4 over
  compat_sys_signalfd.
  (syscall.ustat32): Merged the 2 probes into 1.
* testsuite/buildok/syscalls2-detailed.stp: New testcase.