libcxx: prefer __has_feature(capabilities) to defined(__CHERI__)

Author: brooksdavis

libcxx/include/limits

@@ -448,7 +448,7 @@ protected:
   static _LIBCPP_CONSTEXPR const float_round_style round_style = round_to_nearest;
 };
 
-#ifdef __CHERI__
+#if __has_feature(capabilities)
 template<>
 class __libcpp_numeric_limits<__intcap_t, true>
   : public __libcpp_numeric_limits<long, true> { };