Add compiler support for LLVM's x86 ERMSB feature

This change is needed for compiler-builtins to check for this feature
when implementing memcpy/memset. See:
  https://github.com/rust-lang/compiler-builtins/pull/365

The change just does compile-time detection. I think that runtime
detection will have to come in a follow-up CL to std-detect.

Like all the CPU feature flags, this just references #44839

Signed-off-by: Joe Richey <joerichey@google.com>

compiler/rustc_span/src/symbol.rs

@@ -467,6 +467,7 @@ symbols! {
         encode,
         env,
         eq,
+        ermsb_target_feature,
         err,
         exact_div,
         except,