Keeping nocommon overrides

Set USE=keep-nocommon on sys-config/ltoize to keep the old -fno-common overrides. These were removed with the move to GCC 10. However, they may still be of interest for some users. ltoize will install a symlink next to ltoworkarounds.conf called nocommon.conf when this flag is enabled containing the old overrides. This flag is not enabled by default.