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/inc/hsa.h

@@ -482,19 +482,24 @@ typedef enum {
   */
   HSA_AMD_SYSTEM_INFO_BUILD_VERSION = 0x200,
   /**
-  * Returns true if hsa_amd_svm_* APIs are supported by the driver.  The type of
-  * this attribute is bool.
-  */
+   * Returns true if hsa_amd_svm_* APIs are supported by the driver.  The type of
+   * this attribute is bool.
+   */
   HSA_AMD_SYSTEM_INFO_SVM_SUPPORTED = 0x201,
   // TODO: Should this be per Agent?
   /**
-  * Returns true if all Agents have access to system allocated memory (such as
-  * that allocated by mmap, malloc, or new) by default.
-  * If false then system allocated memory may only be made SVM accessible to
-  * an Agent by declaration of accessibility with hsa_amd_svm_set_attributes.
-  * The type of this attribute is bool.
-  */
-  HSA_AMD_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT = 0x202
+   * Returns true if all Agents have access to system allocated memory (such as
+   * that allocated by mmap, malloc, or new) by default.
+   * If false then system allocated memory may only be made SVM accessible to
+   * an Agent by declaration of accessibility with hsa_amd_svm_set_attributes.
+   * The type of this attribute is bool.
+   */
+  HSA_AMD_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT = 0x202,
+  /**
+   * Returns true if mwaitx is enabled on this system
+   * The type of this attribute is bool.
+   */
+  HSA_AMD_SYSTEM_INFO_MWAITX_ENABLED = 0x203
 } hsa_system_info_t;
 
 /**