Use mwaitx when busy-waiting signals

Use mwaitx instructions when busy waiting for signals to reduce CPU
energy usage.
This can be disabled by setting HSA_ENABLE_MWAITX=0

Change-Id: Ic207895a491b2bf6dacba47ef0921df3faad5b5a

runtime/hsa-runtime/core/inc/runtime.h

@@ -86,6 +86,7 @@ namespace AMD {
 
 namespace core {
 extern bool g_use_interrupt_wait;
+extern bool g_use_mwaitx;
 
 /// @brief  Runtime class provides the following functions:
 /// - open and close connection to kernel driver.