ext/mysql/cas_event_mutex: fix stxr register allocation

Apply earlyclobber constraint on stxr status operand to avoid warnings such as:

/tmp/ccdYBT4m.s:2175: Warning: unpredictable: identical transfer and status registers --`stxr w0,x1,[x0]'
/tmp/ccdYBT4m.s:2716: Warning: unpredictable: identical transfer and status registers --`stxr w0,x1,[x0]'
/tmp/ccdYBT4m.s:4556: Warning: unpredictable: identical transfer and status registers --`stxr w3,x3,[x0]'

Author: jty2

ext/mysql/cas_event_mutex.h

@@ -84,7 +84,7 @@ void lock_signal(void)
                uint32_t exResult;
 
                __asm__ __volatile__ ("stxr %w[exResult], %[lockValue],[%[lockAddr]]"
-                                       : [exResult] "=r" (exResult)
+                                       : [exResult] "=&r" (exResult)
                                        : [lockAddr] "r" (lock), [lockValue] "r" (MUTEX_STATE_LOCKED)
                                        :"memory");