Generated cross-compiler APKBUILDs depend on removed isl package

gcc-aarch64 (and presumably equivalents for other architectures) is currently uninstallable due to missing dependency isl. This blocks setting up a cross-build chroot, so it's a critical problem. isl has been removed as @psykose described in https://gitlab.com/postmarketOS/pmaports/-/issues/1689, apparently the current replacement is isl25.

gcc-aarch64 APKBUILD line with the problem as an example: https://gitlab.com/postmarketOS/pmaports/-/blob/979cc0bfb68a1def9e29b5dc7fb94fa314565a70/cross/gcc-aarch64/APKBUILD#L40