Fix DTrace build on Illumos

DTrace was recently patched in Illumos to fail to create an object
file if no probes are found.

* https://www.illumos.org/issues/4248
* https://github.com/illumos/illumos-gate/commit/54a20ab41aadcb81c53e72fc65886e964e9add59

This patch fixes two issues:

* Modify the configure script to pass an object file to `dtrace -G`
  that actually invokes a probe.

* Remove creation of `dtrace_user.o` from the dyntrace Makefile. In a
  previous commit [1] Scott Fritchie relocated all the user probes
  into the VM proper due to difficulties with DTrace probes in shared
  libraries. The `dtrace_user.d` file is now empty and generates a
  header file with nothing in it. There is no longer any reason to
  generate `dtrace_user.o` because all the probes are in the VM. Thus
  all the steps for building `dtrace_user.o` have been removed.

[1]: https://github.com/erlang/otp/commit/75552bd3bb4e7f3cf4dab81a5c81cf73b1d3fb99