When the diff archiver mode is enabled, the recipes sharing a workdir (gcc and kernel) could fail the task do_unpack_and_patch and/or do_ar_configured. This is because the work dir is modified by another recipe running these task while creating the tarballs.
This is happening only for gcc recipes. It is pointless to archive the gcc source for every recipe using gcc.
Sent patch to the mailing list: http://lists.openembedded.org/pipermail/openembedded-core/2016-March/119480.html
This has been solved in commit: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=9ee403b31067895695f0143b716187d7262c560c