Ages ago this was discussed and at this time, if I remember, -march=native seemed the safer flag and providing better performance. But at this time, Ingo did not yet drastically improved the performance and stability of code.
Furthermore, as I build on a Prescoott hardware, only very very old hardware will be unable to run the code (those missing mmx, sse and sse2).
With mtune=generic, I don't know what is the target hw for which GCC6.1 is supposed to optimize the code. Is it still old i686 hw ?
Though it seems nobody complained for years about impossibility to run the build, if you think it worth, I can provide build with mtune=generic.
Of course, for those building on recent processor, the choice of march =native is incompatible with the distribution of build.