[runtime] New profiler API.

[runtime] New profiler API.

* Profiler callbacks can now be changed and disabled at any point.
* API users no longer have to set event flags. The API instead uses a counter
  internally for each type of callback.
* Filter functions for enter/leave instrumentation can be installed, and they
  can choose whether to instrument the prologue, epilogue, or both.
* Managed allocators can now be instrumented for allocation profiling.
* A profiler must now declare that it wishes to use allocation profiling and/or
  sampling in its init function.
* Sampling parameters can now be changed at any point, and the sampling thread
  can be put into an idle mode when no sampling is needed.
* Only one profiler can have control over the sampling parameters. Whichever
  profiler enables sampling first gets control.
* Adding new events is now very easy: One line in profiler-events.h and one
  line wherever the event should be raised.
* Lifted the restriction that enter/leave instrumentation would cause an abort
  in full AOT mode.
* Support for call chain sampling has been removed.
* Support for the old, platform-specific code coverage mode has been removed.
* The new profiler module entry point is mono_profiler_init. If a module has
  a mono_profiler_startup symbol (the old entry point), a warning will be
  printed and the module won't be loaded.
* Updated the profiler test suite to work with instrumented managed allocators.