Rollup merge of rust-lang#61705 - petrhosek:llvm-cflags, r=alexcrichton

Pass cflags rather than cxxflags to LLVM as CMAKE_C_FLAGS

We mistakenly pass cxxflags from the configuration to LLVM build as
CMAKE_C_FLAGS.

src/bootstrap/native.rs

@@ -416,7 +416,7 @@ fn configure_cmake(builder: &Builder<'_>,
 
  cfg.build_arg("-j").build_arg(builder.jobs().to_string());
  let mut cflags = builder.cflags(target, GitRepo::Llvm).join(" ");
- if let Some(ref s) = builder.config.llvm_cxxflags {
+ if let Some(ref s) = builder.config.llvm_cflags {
  cflags.push_str(&format!(" {}", s));
  }
  cfg.define("CMAKE_C_FLAGS", cflags);