vm_method.c: add new ruby::method-cache-clear dtrace probe

* vm_method.c (rb_clear_method_cache_by_class): fire
  ruby::method-cache-clear probe on global or klass-level method cache
  clear [Bug #9190]
* probes.d (provider ruby): new dtrace probe
* doc/dtrace_probes.rdoc: docs for new probe
* test/dtrace/test_method_cache.rb: test for new probe

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44103 b2dd03c8-39d4-4d8f-98ff-823fe69b080e