linux_5_7: init at 5.7.1

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.