Rollup merge of rust-lang#50104 - mixi:libxml2-llvm, r=alexcrichton

Disable auto-detection of libxml2 when compiling llvm.

This broke cross-compiling rustc with internal llvm (with both the host and target being executable on the machine), because llvm's build system detected libxml2 on the host, therefore auto-enabled libxml2 support, but wouldn't compile as the target didn't have libxml2 installed.

src/bootstrap/native.rs

@@ -149,6 +149,7 @@ impl Step for Llvm {
            .define("WITH_POLLY", "OFF")
            .define("LLVM_ENABLE_TERMINFO", "OFF")
            .define("LLVM_ENABLE_LIBEDIT", "OFF")
+           .define("LLVM_ENABLE_LIBXML2", "OFF")
            .define("LLVM_PARALLEL_COMPILE_JOBS", builder.jobs().to_string())
            .define("LLVM_TARGET_ARCH", target.split('-').next().unwrap())
            .define("LLVM_DEFAULT_TARGET_TRIPLE", target);