terminal adapter: make skipping of initial SIGSTOPs more reliable

first, _start being resolvable depends on libc-dbg being installed.
second, depending on the frame being in the dynloader makes it
a) work only for dynamic executables and b) fail on multi-target
systems (due to a hard-coded file name).
so instead just remember the entry point, as we are already there
anyway.

Reviewed-By: hjk

src/plugins/debugger/gdb/gdbengine.h

@@ -294,6 +294,10 @@ private: ////////// Inferior Management //////////
     void handleInferiorPidChanged(qint64 pid) { manager()->notifyInferiorPidChanged(pid); }
     void maybeHandleInferiorPidChanged(const QString &pid);
 
+#ifdef Q_OS_LINUX
+    QByteArray m_entryPoint;
+#endif
+
 private: ////////// View & Data Stuff //////////
 
     virtual void selectThread(int index);