Adding vmlinux to linux kernel 'dev' derivation.

It takes some extra 13MB (and in dev, not out), but allows perf to show kernel
symbols when profiling. I think it is worth it.

In my NixOS, I refer to it in the system derivation, for easy telling to perf
through /run/booted-system/vmlinux:

system.extraSystemBuilderCmds = ''
ln -s ${config.boot.kernelPackages.kernel.dev}/vmlinux $out/vmlinux
'';

(cherry picked from commit 53a45825523566b8c9c5ff135e8ff75e05558d85)

pkgs/os-specific/linux/kernel/manual-config.nix

···
       buildFlags = [
         "KBUILD_BUILD_VERSION=1-NixOS"
         platform.kernelTarget
+        "vmlinux"  # for "perf" and things like that
       ] ++ optional isModular "modules";
 
       installFlags = [
···
                           if platform.kernelTarget == "zImage" then "zinstall" else
                           "install") ];
 
-      postInstall = (optionalString installsFirmware ''
+      postInstall = ''
+        mkdir -p $dev
+        cp $buildRoot/vmlinux $dev/
+      '' + (optionalString installsFirmware ''
         mkdir -p $out/lib/firmware
       '') + (if (platform ? kernelDTB && platform.kernelDTB) then ''
  	make $makeFlags "''${makeFlagsArray[@]}" dtbs