Don't find absolute path from /proc/self/exe

Similar to legacy/trac#20283 (moved), Mozilla cleaned up their 13+ year old code for finding the install directory path in [1403366]. Instead of relying on argv[0], they now follow /proc/self/exe on Linux systems (they changed how they handle this on other platforms, too).

I'm not sure if we should revert part of the patch.

commit 4fe745918004db0e36e737f945901650be10c1de
Author: Mike Hommey <mh+mozilla@glandium.org>
Date:   Thu Sep 28 10:37:27 2017 +0900

    Bug 1403366 - Stop requiring argv[0] for XRE_GetBinaryPath and the underlying BinaryPath::Get. r=froydnj
    
    --HG--
    extra : rebase_source : ac7bb0d7766fce5b6325c34c2868b26eec594e18