[PATCH 0/3] Merge in gdb default_target_compile changes

This series merges in gdb's changes to default_target_compile,
updating them to follow newer conventions at the same time.

I tested this using various parts of the gdb test suite (in particular
Go and Rust).  I also verified that, with this series in place, gdb
does not use its monkey-patched code from future.exp.

I didn't attempt to further parameterize default_target_compile,
though I think that would be a good thing to do.

Tom