libprobe: Add a probe! macro for static instrumentation

This adds a new libprobe with a probe! macro, for adding static
instrumentation to any rust crate.  Please see the docstrings for simple
probe examples and how they might be used by tools.

This initial version only implements SystemTap SDT, with a voided macro
for other platforms, so probes can be freely written anywhere.  I
believe the macro is generic enough that a DTrace USDT probe! could be
implemented for freebsd and macos too.

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This is related to issue #6816 requesting DTrace USDT, although I only implemented SystemTap SDT.  I enabled this for both Linux and Android target_os, although I only tested the former.  Since it's simply void everywhere else, it's safe to insert probes in any code.

Please let me know if anything needs to change, and I'm also happy to answer questions about debugging with SDT.  Thanks!