pkgs/stdenv/linux: add mips64el bootstrap-files

This PR adds Hydra-generated bootstrap tarballs for mips64el-linux.  I'll be following the script established in https://github.com/NixOS/nixpkgs/pull/151399, which I previously used in https://github.com/NixOS/nixpkgs/pull/168199.

Files came from [this](https://hydra.nixos.org/build/182757245) Hydra build, which used nixpkgs revision ef3fe254f3c59455386bc92fe84164f9679b92b1 to instantiate:

```
/nix/store/a2bvv663wjnyhq8m7v84aspsd3sgf9h6-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64.drv
```

and then built:

```
/nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64
```

I downloaded these files from Hydra with the following commands:

```
STOREPATH=aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64
OPTIONS="--option binary-caches https://cache.nixos.org"
nix store add-file \
  --name bootstrap-tools.tar.xz  \
  $(nix-store ${OPTIONS} -r /nix/store/${STOREPATH})/on-server/bootstrap-tools.tar.xz
nix store add-path \
  --name busybox \
  $(nix-store ${OPTIONS} -r /nix/store/${STOREPATH})/on-server/busybox
```

I then prefetched them into `/nix/store` with:

```
$ nix store prefetch-file --executable file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/busybox
Downloaded 'file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/busybox' to '/nix/store/ai30ss23914syz6j8m95arkwffbbx44k-busybox' (hash 'sha256-sTE58ofjqAqX3Xtq1g9wDxzIe6Vo//GHbicfqJoivDI=').

$ nix store prefetch-file  file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/bootstrap-tools.tar.xz
Downloaded 'file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/bootstrap-tools.tar.xz' to '/nix/store/nr6zjrbwbxcxs6brf042zsyqllwbwj9v-bootstrap-tools.tar.xz' (hash 'sha256-tTgjeXpd2YgnfP4JvRuO0bXd2j8GqzBcd57JI3wH9x0=').
```

And started the bootstrap with the following command (the `--arg localSystem` is needed because #161159 has not merged):

```
nix build -f . -L hello --arg localSystem '(import ./lib).systems.examples.mips64el-linux-gnuabi64'
```

As @lovesegfault requested in #151399, here are the the `sha256sum`s of all the `on-server` components for extra verification:

```
$ sha256sum /nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/*
b53823797a5dd988277cfe09bd1b8ed1b5ddda3f06ab305c779ec9237c07f71d  /nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/bootstrap-tools.tar.xz
598e05abb69b2c1a0db46585cd2131212077c0937ce2a665daf3811f059ae767  /nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/busybox
```

2 files changed, 26 insertions, 0 deletions

diff --git a/pkgs/stdenv/linux/bootstrap-files/mips64el.nix b/pkgs/stdenv/linux/bootstrap-files/mips64el.nix
new file mode 100644
index 0000000000000..452a247fb0b9c
--- /dev/null
+++ b/pkgs/stdenv/linux/bootstrap-files/mips64el.nix
@@ -0,0 +1,25 @@
+#
+# Files came from this Hydra build:
+#
+#   https://hydra.nixos.org/build/182757245
+#
+# Which used nixpkgs revision ef3fe254f3c59455386bc92fe84164f9679b92b1
+# to instantiate:
+#
+#   /nix/store/a2bvv663wjnyhq8m7v84aspsd3sgf9h6-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64.drv
+#
+# and then built:
+#
+#   /nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64
+#
+{
+  busybox = import <nix/fetchurl.nix> {
+    url = "http://tarballs.nixos.org/stdenv-linux/mips64el/ef3fe254f3c59455386bc92fe84164f9679b92b1/busybox";
+    sha256 = "sha256-sTE58ofjqAqX3Xtq1g9wDxzIe6Vo//GHbicfqJoivDI=";
+    executable = true;
+  };
+  bootstrapTools =import <nix/fetchurl.nix> {
+    url = "http://tarballs.nixos.org/stdenv-linux/mips64el/ef3fe254f3c59455386bc92fe84164f9679b92b1/bootstrap-tools.tar.xz";
+    sha256 = "sha256-tTgjeXpd2YgnfP4JvRuO0bXd2j8GqzBcd57JI3wH9x0=";
+  };
+}
diff --git a/pkgs/stdenv/linux/default.nix b/pkgs/stdenv/linux/default.nix
index 6475d7f2ca1e1..874e39fffee16 100644
--- a/pkgs/stdenv/linux/default.nix
+++ b/pkgs/stdenv/linux/default.nix
@@ -16,6 +16,7 @@
       armv7l-linux = import ./bootstrap-files/armv7l.nix;
       aarch64-linux = import ./bootstrap-files/aarch64.nix;
       mipsel-linux = import ./bootstrap-files/loongson2f.nix;
+      mips64el-linux = import ./bootstrap-files/mips64el.nix;
       powerpc64le-linux = import ./bootstrap-files/powerpc64le.nix;
       riscv64-linux = import ./bootstrap-files/riscv64.nix;
     };