linux: Add 5.10

Validated via
```
 nix-build ./nixos/release.nix -A tests.kernel-latest.x86_64-linux -A tests.latestKernel.login.x86_64-linux
```

pkgs/os-specific/linux/kernel/linux-5.10.nix

@@ -0,0 +1,18 @@
+{ stdenv, buildPackages, fetchurl, perl, buildLinux, modDirVersionArg ? null, ... } @ args:
+
+with stdenv.lib;
+
+buildLinux (args // rec {
+  version = "5.10";
+
+  # 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 = "sha256-3N+Z5D6YMw2SUBaYW/vHuDxm02e3FLLeDLv8v4PYykM=";
+  };
+} // (args.argsOverride or {}))

pkgs/top-level/all-packages.nix

@@ -18382,6 +18382,14 @@ in
     ];
   };
 
+  linux_5_10 = callPackage ../os-specific/linux/kernel/linux-5.10.nix {
+    kernelPatches = [
+      kernelPatches.bridge_stp_helper
+      kernelPatches.request_key_helper
+      kernelPatches.export_kernel_fpu_functions."5.3"
+    ];
+  };
+
   linux-rt_5_9 = callPackage ../os-specific/linux/kernel/linux-rt-5.9.nix {
     kernelPatches = [
       kernelPatches.bridge_stp_helper
@@ -18635,7 +18643,7 @@ in
 
   # Update this when adding the newest kernel major version!
   # And update linux_latest_for_hardened below if the patches are already available
-  linuxPackages_latest = linuxPackages_5_9;
+  linuxPackages_latest = linuxPackages_5_10;
   linux_latest = linuxPackages_latest.kernel;
 
   # Realtime kernel packages.
@@ -18660,6 +18668,7 @@ in
   linuxPackages_5_4 = recurseIntoAttrs (linuxPackagesFor pkgs.linux_5_4);
   linuxPackages_5_8 = recurseIntoAttrs (linuxPackagesFor pkgs.linux_5_8);
   linuxPackages_5_9 = recurseIntoAttrs (linuxPackagesFor pkgs.linux_5_9);
+  linuxPackages_5_10 = recurseIntoAttrs (linuxPackagesFor pkgs.linux_5_10);
 
   # When adding to the list above:
   # - Update linuxPackages_latest to the latest version