FFe: update gccgo-6 to the GCC 6.1 release candidate 1

[Matthias Klose]

The aim is to update gccgo-6 to the final release 6.1 in 16.04 with two steps:

 - update to the 6.1 release candidate 1 (6.0.1) before releae
 - update to 6.1.0 in xenial-proposed/updates after release.

Compared to the version in xenial (20160405), there are no source changes in libgcc, libgo and gcc/go. During this stage gcc upstream was in stage4, and only saw regression fixes.

In main, libgcc (built by gccgo-6) is used on every architecture, gccgo is only used on powerpc. The package was test-built in a ppa on all architectures, not showing regressions in it's testsuite. Installing these packages on a amd64 desktop works fine, the system survives a reboot and continues to work as expected.

https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/ppa/+sourcepub/6309376/+listing-archive-extra

[Adam Conrad (adconrad)]

Looks reasonableish to me. Accepting.