rewrite preinit as very small C program

By using the kernel "nolibc" header to avoid requiring a C library, we
can bring the initramfs size to around 4k

This does involve a tiny bit of inline mips assembly which I'm not
sure about. gcc seems unwilling to generate the code to load $gp at
function entry of main(), so we do it by hand - but I'd rather find
out why gcc doesn't.

pkgs/preinit/default.nix

@@ -0,0 +1,30 @@
+{
+  stdenv
+, fetchzip
+, gdb
+ }:
+let kernel = fetchzip {
+      name = "linux";
+      url = "https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.71.tar.gz";
+      hash = "sha256-pq6QNa0PJVeheaZkuvAPD0rLuEeKrViKk65dz+y4kqo=";
+    };
+in
+stdenv.mkDerivation {
+  name = "preinit";
+  src = ./.;
+
+  #  NIX_DEBUG=2;
+  hardeningDisable = [ "all" ];
+  CFLAGS = "-Os  -nostartfiles -nostdlib  -fno-stack-protector -fpic -fPIC  -I ./ -I ${kernel}/tools/include/nolibc";
+
+  postBuild = ''
+    $STRIP  --remove-section=.note --remove-section=.comment preinit
+  '';
+
+  makeFlags = ["preinit"];
+  stripAllList = ["bin"];
+  installPhase = ''
+    mkdir -p $out/bin
+    cp preinit $out/bin
+  '';
+}