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.

pkgs/os-specific/linux/kernel/linux-5.7.nix

@@ -0,0 +1,18 @@
+{ stdenv, buildPackages, fetchurl, perl, buildLinux, modDirVersionArg ? null, ... } @ args:
+
+with stdenv.lib;
+
+buildLinux (args // rec {
+  version = "5.7.1";
+
+  # modDirVersion needs to be x.y.z, will automatically add .0 if needed
+  modDirVersion = if (modDirVersionArg == null) then concatStringsSep "." (take 3 (splitVersion "${version}.0")) else modDirVersionArg;
+
+  # branchVersion needs to be x.y
+  extraMeta.branch = versions.majorMinor version;
+
+  src = fetchurl {
+    url = "mirror://kernel/linux/kernel/v5.x/linux-${version}.tar.xz";
+    sha256 = "1vcxrrb2i4366iciw0mfahwbdrzmhrrsr7gi4vdkzznfv2niils0";
+  };
+} // (args.argsOverride or {}))