[sgen] Move DTrace support to Mono-specific code.

[sgen] Move DTrace support to Mono-specific code.

We use client callbacks from the binary protocol functions.  There are some DTrace probes
which we don't have binary protocol entries for.  Since DTrace isn't properly supported
anyway (and doesn't even work as it should on OSX), we just remove those probes instead
of making new binary protocol entries.