lib.systems.examples: add UEFI examples

These are mostly intended for use with Rust, but that involves
building compiler-rt for the cross system, so we are capable of
building C/C++ as well.

lib/systems/examples.nix

@@ -221,6 +221,32 @@ rec {
     libc = "newlib";
   };
 
+  #
+  # UEFI
+  #
+
+  # See: https://github.com/rust-lang/rust/pull/56769/files#r241916113
+  i686-uefi = {
+    system = "x86_64-windows";
+    rust.config = "i686-unknown-uefi";
+    useLLVM = true;
+    libc = null;
+  };
+
+  x86_64-uefi = {
+    system = "x86_64-windows";
+    rust.config = "x86_64-unknown-uefi";
+    useLLVM = true;
+    libc = null;
+  };
+
+  aarch64-uefi = {
+    system = "aarch64-windows";
+    rust.config = "aarch64-unknown-uefi";
+    useLLVM = true;
+    libc = null;
+  };
+
   #
   # Redox
   #