…by < 2.6

By relying on the `debase-ruby_core_source` extension, we can get access
to the internal VM headers needed to implement this functionality on
all legacy Ruby versions we support.

Note that this does not yet change the profiler to use
`NativeExtension.clock_id_for` on these older Rubies.

This abstracts away the actual operation we want in our code -- getting
the cpu time for a given thread, rather than exposing the clock_id
and having callers handle it.

We now gather cpu time directly using the native extension, rather
than relying on the `cthread.rb` monkey patch.

This allows us to remove `cthread.rb` (next commit), as well as to
clean up the whole "let's try to detect when there's missing
instrumentation" approach we had going on.

It also enables us to simpify the tests. All great advantages!

We want to preserve this knowledge and tests, so before deleting
`cthread.rb` and its spec, let's move this to the stack sampler.

After we move CPU time to use the native extension, we no longer need
the "CPU extension" that monkey patches `Thread` creation.

It's been fully replaced by our usage of the native extension to get
this information.

`ffi` was only used by the profiling CPU time extension (`cthread.rb`)
which has been removed so let's get rid of it. Thanks `ffi` for your
service.

We still have a lot of other Windows-related failures, so let's get
back to this once 3.1 is released.

Hopefully we can deprecate Ruby 2.1 soon.

… review