propagate build.python into cmake

If a suitable value of Python is not on PATH, one can still invoke x.py
manually, which propagates BOOTSTRAP_PYTHON into the bootstrap
environment.  But building LLVM will abort with error messages about not
being able to find Python, and instructions to set PYTHON_EXECUTABLE,
because nothing is done with BOOTSTRAP_PYTHON when invoking cmake.
Setting build.python in config.toml had no effect in this scenario,
either

To fix this, let's provide PYTHON_EXECUTABLE when invoking cmake; for
the "normal" case of Python in PATH, this doesn't alter any behavior.
For more unusual cases, however, this ensures cmake finds Python
properly.  (This change also ensures there are no differences between
what bootstrap is using, and what cmake uses, which may be useful for
consistency's sake.)

src/bootstrap/native.rs

@@ -239,6 +239,10 @@ impl Step for Llvm {
             cfg.define("LLVM_NATIVE_BUILD", builder.llvm_out(builder.config.build).join("build"));
         }
 
+        if let Some(ref python) = builder.config.python {
+            cfg.define("PYTHON_EXECUTABLE", python);
+        }
+
         configure_cmake(builder, target, &mut cfg, false);
 
         // FIXME: we don't actually need to build all LLVM tools and all LLVM