Refactor sdt & plt pattern registration

Refactor sdt & plt pattern registration

This factors the sdt and plt suffixes away from the various process and
process.library prefixes.  As a result, we also gain process.plt for
probing -c commands.

* tapsets.cxx (dwarf_derived_probe::register_sdt_variants): New.
  (dwarf_derived_probe::register_plt_variants): New.
  (dwarf_derived_probe::register_patterns): Register sdt and plt for all
  process/library combinations.
* testsuite/semok/plt2.stp: Test that process.plt succeeds with -c
* testsuite/semko/plt2.stp: Test that process.plt fails without -c