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.
Merge pull request #4089

74fc254 devtools: add script to check symbols from Linux gitian executables (Wladimir J. van der Laan)