cross/gcc-*: add fixme note for the isl workaround

Make sure we drop !tracedeps next time when upgrading the gcc packages
(after Alpine's gcc has been upgraded). The workaround we have in
pmbootstrap is tied to the current version, and that's the only way we
can remove it for good. The FIXME line can be deleted after this has
been done. Note that current 'pmbootstrap aportgen gcc-*' code will
automatically remove the !tracedeps option - a reminder to double check
it.

Related: #1732 (closed)

Merging after CI passes, this is just a comment.