Rollup merge of #76472 - matthiaskrgr:llvm_cmake_vars, r=Mark-Simulacrum

rustbuild: don't set PYTHON_EXECUTABLE and WITH_POLLY cmake vars since they are no longer supported by llvm

This resolves

CMake Warning:
  Manually-specified variables were not used by the project:

    PYTHON_EXECUTABLE
    WITH_POLLY

src/bootstrap/native.rs

@@ -169,7 +169,6 @@ impl Step for Llvm {
             .define("LLVM_INCLUDE_TESTS", "OFF")
             .define("LLVM_INCLUDE_DOCS", "OFF")
             .define("LLVM_INCLUDE_BENCHMARKS", "OFF")
-            .define("WITH_POLLY", "OFF")
             .define("LLVM_ENABLE_TERMINFO", "OFF")
             .define("LLVM_ENABLE_LIBEDIT", "OFF")
             .define("LLVM_ENABLE_BINDINGS", "OFF")
@@ -305,10 +304,6 @@ impl Step for Llvm {
             cfg.define("LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN", "YES");
         }
 
-        if let Some(ref python) = builder.config.python {
-            cfg.define("PYTHON_EXECUTABLE", python);
-        }
-
         configure_cmake(builder, target, &mut cfg, true);
 
         // FIXME: we don't actually need to build all LLVM tools and all LLVM