rebootstrap: add gcc-6

This adds 66 jobs and does not allocate any further resources to those
jobs. They will be run infrequently at first and replace the gcc-5 jobs
in the long run.

job-cfg/rebootstrap.yaml.py

@@ -33,7 +33,7 @@ release_architectures = """
 
 architectures += mono_architectures
 
-gcc_versions = ("5",)
+gcc_versions = ("5", "6")
 diffoscope_gcc_versions = ("5",)
 
 print("""