Add support for DTrace

[Jan Vrany]

Smalltalk/X has very basic support for ​SystemTap for some time but we can do better.

We should use probe definition file (.d). This would have several advantages:

• it would make it compatible with *BSD & macOS DTrace
• it would allow to use ..._ENABLED() macros when appropriate.
• probe definition file makes a good place to document which probes are available and what's the meaning of the parameters (if any)

The downside of this is that we'd have to provide our own "fake" implementation of dtrace command to generate empty macros on platforms
with no DTrace support. Ideally this fake implementation should be implemented in Ruby as Ruby is needed to build Smalltalk/X anyway.