Updated documentation, x86 feature detection testing, and removed LLVM 9 exclusive features

Updated the added documentation in llvm_util.rs to note which copies of LLVM need to be inspected.
Removed avx512bf16 and avx512vp2intersect because they are unsupported before LLVM 9 with the build with external LLVM 8 being supported
Re-introduced detection testing previously removed for un-requestable features tsc and mmx

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -139,6 +139,11 @@ pub fn time_trace_profiler_finish(file_name: &str) {
 // array, leading to crashes.
 // To find a list of LLVM's names, check llvm-project/llvm/include/llvm/Support/*TargetParser.def
 // where the * matches the architecture's name
+// Beware to not use the llvm github project for this, but check the git submodule
+// found in src/llvm-project
+// Though note that Rust can also be build with an external precompiled version of LLVM
+// which might lead to failures if the oldest tested / supported LLVM version
+// doesn't yet support the relevant intrinsics
 pub fn to_llvm_feature<'a>(sess: &Session, s: &'a str) -> &'a str {
     let arch = if sess.target.arch == "x86_64" { "x86" } else { &*sess.target.arch };
     match (arch, s) {