Make the global break check volatile

The Suspend debugger test was hanging intermittently on most platforms.
These seemed to occur because the codegen checks for
`g_Il2CppDebuggerCheckPointEnabled` were not getting the proper value,
and assuming the check point was not set, so the VM would not suspend.

This meant that the main thread could not be suspended when the debugger
was trying to exit, so the main thread kept looping forever, with out
allowing the debugger to exit it.

Author: Josh Peterson

mono/mini/debugger-agent.c

@@ -1161,7 +1161,7 @@ void mono_debugger_install_runtime_callbacks(MonoDebuggerRuntimeCallbacks* cbs)
 }
 
 uint32_t* g_unity_check;
-void mono_debugger_install_sequence_point_check(uint32_t* check)
+void mono_debugger_install_sequence_point_check(volatile uint32_t* check)
 {
 	g_unity_check = check;
 }