Rollup merge of rust-lang#91548 - luojia65:hint-spin-loop-riscv, r=Amanieu

Add spin_loop hint for RISC-V architecture

This commit uses the PAUSE instruction (rust-lang/stdarch#1262) to implement RISC-V spin loop, and updates `stdarch` submodule to use the merged PAUSE instruction.

Author: matthiaskrgr

core/src/hint.rs

@@ -137,6 +137,11 @@ pub fn spin_loop() {
             unsafe { crate::arch::arm::__yield() };
         }
     }
+
+    #[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))]
+    {
+        crate::arch::riscv::pause();
+    }
 }
 
 /// An identity function that *__hints__* to the compiler to be maximally pessimistic about what

stdarch

@@ -1 +1 @@
-Subproject commit cfba59fccd90b3b52a614120834320f764ab08d1
+Subproject commit b70ae88ef2a6c83acad0a1e83d5bd78f9655fd05