pkgs/top-level/release-cross.nix: add Hydra jobs for pkgsCross.platform.nixStatic

This commit causes Hydra to build `nixStatic` on non-NixOS platforms
for which nixStatic is known to build correctly.

Providing Hydra builds of `nixStatic` on these platforms allows users
to bootstrap their local nixpkgs system without either having to:

a. Trust binaries that came from a source other than Hydra or

b. Fight with their host distribution to satisfy all of nix's large
set of build dependencies (this is not easy!)

Currently there are two platforms in this set: mips64el-linux-gnuabi64
and powerpc64le-linux-gnu.

pkgs/top-level/release-cross.nix

···
     buildPackages.binutils = nativePlatforms;
     mpg123 = nativePlatforms;
   };
+
+  # Enabled-but-unsupported platforms for which nix is known to build.
+  # We provide Hydra-built `nixStatic` for these platforms.  This
+  # allows users to bootstrap their own system without either (a)
+  # trusting binaries from a non-Hydra source or (b) having to fight
+  # with their host distribution's versions of nix's numerous
+  # build dependencies.
+  nixCrossStatic = {
+    nixStatic = nativePlatforms;
+  };
+
 in
 
 {
···
     # attribute, so there is no way to detect this -- we must add it
     # as a special case.
     (builtins.removeAttrs tools ["bootstrapTools"]);
+
+  # Cross-built nixStatic for platforms for enabled-but-unsupported platforms
+  mips64el-nixCrossStatic = mapTestOnCross lib.systems.examples.mips64el-linux-gnuabi64 nixCrossStatic;
+  powerpc64le-nixCrossStatic = mapTestOnCross lib.systems.examples.powernv nixCrossStatic;
 }