preparing for binutils-2.31

According to [1], binutils 2.31 (currently in experimental) will branch in about 
a week, and I'll plan to upload the branch version to unstable.  Test results 
are reported to [2], these look reasonable, except for the various mips targets, 
however as seen in the past, it doesn't make a difference if you wait with the 
introduction of a new binutils version early or later with the release.  These 
tend to be only fixed as Debian porters report them.

Matthias

[1] https://sourceware.org/ml/binutils/2018-06/msg00158.html
[2] https://sourceware.org/ml/binutils/2018-06/msg00170.html