Fix the named semaphore leak on OSX (and Linux) (dotnet/coreclr#5269)

Fix the named semaphore leak on OSX (and Linux) (dotnet/coreclr#5269)

* Change the dbgshim launch handshake back.

The debugger side now creates the name semaphores like before and the transport pipe
existence determines that coreclr is ready.

Changed when the transport pipes are created: synchronously on the main thread.

Correctly set and check the HAVE_PROCFS_* defines.

* Code review feedback.

Commit migrated from https://github.com/dotnet/coreclr/commit/5ac6af932fe2a3f4b285b6dcc79010caf807ea9d