linuxHeaders: fix cross-compilation from darwin to mips

mips tools require endian headers which are different on darwin.
Add stub headers to define the appropriate byte swap functions.

Co-authored-by: Adam Joseph <[e-mail]>

pkgs/os-specific/linux/kernel-headers/default.nix

···
 { stdenvNoCC, lib, buildPackages, fetchurl, perl, elf-header
 , bison ? null, flex ? null, python ? null, rsync ? null
+, writeTextFile
 }:
 
 assert stdenvNoCC.hostPlatform.isAndroid ->
   (flex != null && bison != null && python != null && rsync != null);
 
 let
+
+  # As part of building a hostPlatform=mips kernel, Linux creates and runs a
+  # tiny utility `arch/mips/boot/tools/relocs_main.c` for the buildPlatform.
+  # This utility references a glibc-specific header `byteswap.h`.  There is a
+  # compatibility header in gnulib for most BSDs, but not for Darwin, so we
+  # synthesize one here.
+  darwin-endian-h = writeTextFile {
+    name = "endian-h";
+    text = ''
+      #include <byteswap.h>
+    '';
+    destination = "/include/endian.h";
+  };
+  darwin-byteswap-h = writeTextFile {
+    name = "byteswap-h";
+    text = ''
+      #pragma once
+      #include <libkern/OSByteOrder.h>
+      #define bswap_16 OSSwapInt16
+      #define bswap_32 OSSwapInt32
+      #define bswap_64 OSSwapInt64
+    '';
+    destination = "/include/byteswap.h";
+  };
+
   makeLinuxHeaders = { src, version, patches ? [] }: stdenvNoCC.mkDerivation {
     inherit src;
 
···
       perl elf-header
     ] ++ lib.optionals stdenvNoCC.hostPlatform.isAndroid [
       flex bison python rsync
+    ] ++ lib.optionals (stdenvNoCC.buildPlatform.isDarwin &&
+                        stdenvNoCC.hostPlatform.isMips) [
+      darwin-endian-h
+      darwin-byteswap-h
     ];
 
     extraIncludeDirs = lib.optional (with stdenvNoCC.hostPlatform; isPower && is32bit && isBigEndian) ["ppc"];