Add is_riscv_feature_detected!; modify impl of hint::spin_loop

Update library/core/src/hint.rs

Co-authored-by: Amanieu d'Antras <amanieu@gmail.com>

Remove redundant config gate

library/std/src/lib.rs

@@ -556,6 +556,7 @@ pub use std_detect::*;
 pub use std_detect::{
     is_aarch64_feature_detected, is_arm_feature_detected, is_mips64_feature_detected,
     is_mips_feature_detected, is_powerpc64_feature_detected, is_powerpc_feature_detected,
+    is_riscv_feature_detected,
 };
 
 // Re-export macros defined in libcore.