GDB: Fix display of disassembly with optimizations

/m flag is deprecated since 7.11. Use /s instead.

See https://sourceware.org/gdb/onlinedocs/gdb/Machine-Code.html

Change-Id: Ie056bcf598da30976fca1622bc98eeb79a73f8e1
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/gdb/gdbengine.h

@@ -403,6 +403,7 @@ private: ////////// General Interface //////////
     void shutdownEngine() final;
 
     void interruptInferior2();
+    QChar mixedDisasmFlag() const;
 
     // Plain
     void handleExecRun(const DebuggerResponse &response);