x64: cpu_wait: Implement MWAITX for non-MSVC compilers

src/common/x64/cpu_wait.cpp

@@ -47,6 +47,16 @@ static void TPAUSE() {
     const auto edx = static_cast<u32>(tsc >> 32);
     asm volatile("tpause %0" : : "r"(RequestC02State), "d"(edx), "a"(eax));
 }
+
+static void MWAITX() {
+    static constexpr auto EnableWaitTimeFlag = 1U << 1;
+    static constexpr auto RequestC1State = 0U;
+
+    // monitor_var should be aligned to a cache line.
+    alignas(64) u64 monitor_var{};
+    asm volatile("monitorx" : : "a"(&monitor_var), "c"(0), "d"(0));
+    asm volatile("mwaitx" : : "a"(RequestC1State), "b"(PauseCycles), "c"(EnableWaitTimeFlag));
+}
 #endif
 
 void MicroSleep() {