tracepoints: Work with the tracequery's .o rather than .ko

tracepoints: Work with the tracequery's .o rather than .ko

The intermediate object files are slightly smaller, since they don't
have any of the module boilerplate compiled in yet.  It still has what
we need though, just the basic stapprobe_* function definitions.

* buildrun.cxx (make_tracequeries): Return .o filenames rather than .ko.
* hash.cxx (find_tracequery_hash): Use .o for cached filenames.
* tapsets.cxx (tracepoint_builder::get_tracequery_modules): Update
  comments and variable names to refer to objects instead of kos.