cpufeatures: cpuid isn't supported under miri

Miri is an interpreter and doesn't support inline assembly.
Fortunately, we already have precedent for an x86 target without
cpuid: SGX. Treat miri like SGX: only features that are enabled at
compile time are available at run time.

Author: jrose-signal

cpufeatures/src/x86.rs

@@ -6,19 +6,19 @@
 // Evaluate the given `$body` expression any of the supplied target features
 // are not enabled. Otherwise returns true.
 //
-// The `$body` expression is not evaluated on SGX targets, and returns false
-// on these targets unless *all* supplied target features are enabled.
+// The `$body` expression is not evaluated on SGX targets or under Miri, and
+// returns false on these targets unless *all* supplied features are enabled.
 #[macro_export]
 #[doc(hidden)]
 macro_rules! __unless_target_features {
     ($($tf:tt),+ => $body:expr ) => {{
         #[cfg(not(all($(target_feature=$tf,)*)))]
         {
-            #[cfg(not(target_env = "sgx"))]
+            #[cfg(not(any(miri, target_env = "sgx")))]
             $body
 
-            // CPUID is not available on SGX targets
-            #[cfg(target_env = "sgx")]
+            // CPUID is not available in certain configurations.
+            #[cfg(any(miri, target_env = "sgx"))]
             false
         }