linuxManualConfig: install GDB scripts

These are required to debug kernel modules.  Since we're now able to
do that, there's another reason besides BTF to enable DEBUG_INFO, so
I've done that for pre-BTF kernel modules as well here.

For GDB to get configured correctly, vmlinux-gdb.py has to be two
directories up from scripts/gdb, and vmlinux has to be next to
vmlinux-gdb.py.  The least invasive way to satisfy these constraints
is to make vmlinux a symlink, which GDB will resolve before looking
for vmlinux-gdb.py.

Tested both ways of getting the scripts into GDB that I know of:

gdb /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/vmlinux \
    -iex 'add-auto-load-safe-path /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/lib/modules/6.1.19/build/vmlinux-gdb.py' \
    -ex 'lx-version' \
    -ex 'q'
gdb /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/vmlinux \
    -ex 'source /nix/store/7n77ijlxkxr6d613h02lr707kvjx6j1k-linux-6.1.19-dev/lib/modules/6.1.19/build/vmlinux-gdb.py' \
    -ex 'lx-version' \
    -ex 'q'

Also tested that the strip changes don't result in meaningful output
size changes (there's some small variation due to BTF data not always
coming out the same size, which is unrelated), and built every kernel
I can on x86_64 to make sure I'm not relying on build system behaviour
specific to newer kernels.

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

@@ -36,10 +36,7 @@ let
 
     debug = {
       # Necessary for BTF
-      DEBUG_INFO                = mkMerge [
-        (whenOlder "5.2" (if (features.debug or false) then yes else no))
-        (whenBetween "5.2" "5.18" yes)
-      ];
+      DEBUG_INFO                = yes;
       DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT = whenAtLeast "5.18" yes;
       # Reduced debug info conflict with BTF and have been enabled in
       # aarch64 defconfig since 5.13
@@ -62,6 +59,8 @@ let
       SUNRPC_DEBUG              = yes;
       # Provide access to tunables like sched_migration_cost_ns
       SCHED_DEBUG               = yes;
+
+      GDB_SCRIPTS               = yes;
     };
 
     power-management = {

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

@@ -277,7 +277,6 @@ let
       ];
 
       postInstall = optionalString isModular ''
-        cp vmlinux $dev/
         if [ -z "''${dontStrip-}" ]; then
           installFlagsArray+=("INSTALL_MOD_STRIP=1")
         fi
@@ -297,12 +296,16 @@ let
         # from a `try-run` call from the Makefile
         rm -f $dev/lib/modules/${modDirVersion}/build/.[0-9]*.d
 
-        # Keep some extra files on some arches (powerpc, aarch64)
-        for f in arch/powerpc/lib/crtsavres.o arch/arm64/kernel/ftrace-mod.o; do
-          if [ -f "$buildRoot/$f" ]; then
-            cp $buildRoot/$f $dev/lib/modules/${modDirVersion}/build/$f
+        # Keep some extra files
+        for f in arch/powerpc/lib/crtsavres.o arch/arm64/kernel/ftrace-mod.o \
+                 scripts/gdb/linux vmlinux vmlinux-gdb.py
+        do
+          if [ -e "$buildRoot/$f" ]; then
+            mkdir -p "$(dirname "$dev/lib/modules/${modDirVersion}/build/$f")"
+            cp -HR $buildRoot/$f $dev/lib/modules/${modDirVersion}/build/$f
           fi
         done
+        ln -s $dev/lib/modules/${modDirVersion}/build/vmlinux $dev
 
         # !!! No documentation on how much of the source tree must be kept
         # If/when kernel builds fail due to missing files, you can add
@@ -345,6 +348,11 @@ let
         sed -i Makefile -e 's|= ${buildPackages.kmod}/bin/depmod|= depmod|'
       '';
 
+      preFixup = ''
+        # Don't strip $dev/lib/modules/*/vmlinux
+        stripDebugList="$(cd $dev && echo lib/modules/*/build/*/)"
+      '';
+
       requiredSystemFeatures = [ "big-parallel" ];
 
       meta = {