lib/systems: add microblaze-embedded

Adds a new target for `pkgsCross` that can be used for building
freestanding code for microblaze targets (e.g. Xilinx PMU firmware).

lib/systems/examples.nix

···
     libc = "newlib";
   };
 
+  microblaze-embedded = {
+    config = "microblazeel-none-elf";
+    libc = "newlib";
+  };
+
   #
   # Redox
   #