linux/common-config: fix crash kernel on i686

Following
https://github.com/torvalds/linux/blob/ee94b00c1a648530333d9734200be7a45e6e00cd/Documentation/admin-guide/kdump/kdump.rst#dump-capture-kernel-config-options-arch-dependent-i386-and-x86_64,
we need to set CONFIG_HIGHMEM4G as well (not CONFIG_HIGHMEM64G which we
already set).

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

@@ -111,6 +111,7 @@ let
 
       # Enable crashkernel support
       PROC_VMCORE = yes;
+      HIGHMEM4G = lib.mkIf (stdenv.hostPlatform.isx86 && stdenv.hostPlatform.is32bit) yes;
 
       # Track memory leaks and performance issues related to allocations.
       MEM_ALLOC_PROFILING = whenAtLeast "6.10" yes;