retpoline-enabled GCC build for jessie

[Moritz Mühlenhoff <jmm@inutil.org>]

Hi,
I've created a GCC 4.9 package for jessie with backported support for
-mindirect-branch (as needed to build kernels with retpoline support).
packages are available at https://people.debian.org/~jmm/gcc/. I've run some
tests, but would appreciate additional testing feedback; the update is planned
to be released end of week/weekend.

Cheers,
        Moritz