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_codegen_ssa/src/target_features.rs

@@ -63,6 +63,7 @@ const X86_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     ("bmi1", None),
     ("bmi2", None),
     ("cmpxchg16b", Some(sym::cmpxchg16b_target_feature)),
+    ("ermsb", Some(sym::ermsb_target_feature)),
     ("f16c", Some(sym::f16c_target_feature)),
     ("fma", None),
     ("fxsr", None),