gcc: arc: bump to git commit c6227bc

gcc: arc: bump to git commit c6227bc

This fixes two issues: one segmentation fault in GCC and one invalid code
generation. Those fix numerous autobuild failures, including:
http://autobuild.buildroot.net/results/715/7158a2a19da6bfa950125a951a39061ccaa73101/
http://autobuild.buildroot.net/results/9d4/9d4fbcb91cf76e01c833d70c401c0828ad37631a/
http://autobuild.buildroot.net/results/ef6/ef6a0e2d382ae202bb8f0e9fc9f5e48c90119faf/
http://autobuild.buildroot.net/results/276/27692619efee482cded56967017f260bd30eefe7/

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>