rustc: Fix support for LLVM 3.3

The llvm.copysign and llvm.round intrinsics weren't added until LLVM 3.4, so if
we're on LLVM 3.3 we lower these to calls in libm instead of LLVM intrinsics.

This should fix our travis failures.

src/rustllvm/RustWrapper.cpp

@@ -652,3 +652,8 @@ LLVMRustSetDLLExportStorageClass(LLVMValueRef Value) {
     LLVMSetLinkage(Value, LLVMDLLExportLinkage);
 }
 #endif
+
+extern "C" int
+LLVMVersionMinor() {
+    return LLVM_VERSION_MINOR;
+}