Changes:
- Copied linux-5.7.nix from linux-5.6.nix
- Add linux_5_7 and linuxPackages_5_7
- Update linux_latest to 5.7

Note:
The kernel patch 'kernelPatches.export_kernel_fpu_functions."5.3"' is
still applied as I copied the list from linux_5_7 (vs. linux_testing).
This patch is probably still required for the ZFS performance.

This adds an abstraction/hack to update the latest hardened kernel
independent of linux_latest, which is required as the hardened patches
aren't immediately available ([0] currently doesn't exist).
Currently the Linux hardened patches aren't even available for 5.7 which
was released on 2020-05-31 (already 9 days ago!).
Therefore it is required to keep both linuxPackages_latest_hardened and
linuxPackages_latest_xen_dom0_hardened at 5.6 until the patches for 5.7
are released.

This abstraction makes that task more manageable but we should aim for a
better solution to resolve this or at least a cleaner and more robust
implementation (in the sense of better error messages).

[0]: https://github.com/anthraxx/linux-hardened/releases/tag/5.7.1.a

linux_5_7: init at 5.7.1