Cleanup LLVM multi-threading checks

The support for runtime multi-threading was removed from LLVM. Calls to
`LLVMStartMultithreaded` became no-ops equivalent to checking if LLVM
was compiled with support for threads http://reviews.llvm.org/D4216.

Author: tmiasko

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1743,7 +1743,7 @@ extern "C" {
 
     pub fn LLVMDisposeMessage(message: *mut c_char);
 
-    pub fn LLVMStartMultithreaded() -> Bool;
+    pub fn LLVMIsMultithreaded() -> Bool;
 
     /// Returns a string describing the last error caused by an LLVMRust* call.
     pub fn LLVMRustGetLastError() -> *const c_char;

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -17,35 +17,25 @@ use std::path::Path;
 use std::ptr;
 use std::slice;
 use std::str;
-use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Once;
 
-static POISONED: AtomicBool = AtomicBool::new(false);
 static INIT: Once = Once::new();
 
 pub(crate) fn init(sess: &Session) {
     unsafe {
         // Before we touch LLVM, make sure that multithreading is enabled.
+        if llvm::LLVMIsMultithreaded() != 1 {
+            bug!("LLVM compiled without support for threads");
+        }
         INIT.call_once(|| {
-            if llvm::LLVMStartMultithreaded() != 1 {
-                // use an extra bool to make sure that all future usage of LLVM
-                // cannot proceed despite the Once not running more than once.
-                POISONED.store(true, Ordering::SeqCst);
-            }
-
             configure_llvm(sess);
         });
-
-        if POISONED.load(Ordering::SeqCst) {
-            bug!("couldn't enable multi-threaded LLVM");
-        }
     }
 }
 
 fn require_inited() {
-    INIT.call_once(|| bug!("llvm is not initialized"));
-    if POISONED.load(Ordering::SeqCst) {
-        bug!("couldn't enable multi-threaded LLVM");
+    if !INIT.is_completed() {
+        bug!("LLVM is not initialized");
     }
 }