rustc: Add the cmpxchg16b target feature on x86/x86_64

This appears to be called `cx16` in LLVM and a few other locations, but
the Intel Intrinsic Guide doesn't have a name for this and the CPU
manual from Intel only mentions `cmpxchg16b`, so that's the name chosen
here.

src/librustc_codegen_llvm/llvm_util.rs

@@ -140,6 +140,7 @@ const X86_WHITELIST: &[(&str, Option<&str>)] = &[
     ("avx512vpopcntdq", Some("avx512_target_feature")),
     ("bmi1", None),
     ("bmi2", None),
+    ("cmpxchg16b", Some("cmpxchg16b_target_feature")),
     ("fma", None),
     ("fxsr", None),
     ("lzcnt", None),
@@ -212,6 +213,7 @@ pub fn to_llvm_feature<'a>(sess: &Session, s: &'a str) -> &'a str {
         ("x86", "pclmulqdq") => "pclmul",
         ("x86", "rdrand") => "rdrnd",
         ("x86", "bmi1") => "bmi",
+        ("x86", "cmpxchg16b") => "cx16",
         ("aarch64", "fp") => "fp-armv8",
         ("aarch64", "fp16") => "fullfp16",
         (_, s) => s,