Previously only cross/gcc{4,6}-* packages were adjusted in MR 3419, but
these packages need to be adjusted as well.
Fix for error shown in upstream_compat check:
Increase 'gcc6' pkgrel (10 -> 11), missing depend(s): isl
Increase 'gcc4' pkgrel (5 -> 6), missing depend(s): isl
Related: pmbootstrap issue 2167
[ci:skip-build] Already built successfully once