devtools: add script to check symbols from Linux gitian executables #4089

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 .

MAX_VERSIONS in the script can easily be changed when, at some point, our minimum requirements change.

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/74fc254c2b4b2fd8392a246fc3b1520ca88da4fd for binaries and test log. This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/ Contact BlueMatt on freenode if something looks broken.