Debugger: Enforce use of target-async for Android gdbserver

Since NDK r11 this seems to be needed to get breakpoints set
correctly on Linux and Mac.

Change-Id: I54281feecc0a915d4761228c612b72889756aecf
Reviewed-by: BogDan Vatra <bogdan@kdab.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/debugger/gdb/gdbengine.h

@@ -423,6 +423,7 @@ protected:
     bool m_terminalTrap;
     bool m_temporaryStopPending;
     bool usesExecInterrupt() const;
+    bool usesTargetAsync() const;
 
     QHash<int, QByteArray> m_scheduledTestResponses;
     QSet<int> m_testCases;