devtools: add script to check symbols from Linux gitian executables

Add a script to check that the (Linux) executables produced by gitian only contain allowed gcc, glibc and libstdc++ version symbols.  This makes sure they are still compatible with the minimum supported Linux
distribution versions.

Running this script - ideally it would be part of the nightly builds - makes sure that the work in #4042 is not undone. The list of new, incompatible symbols is immediately shown so that they can be wrapped.

I've tested this against Gitian-produced executables with and without `--enable-glibc-back-compat` and the result is as expected .