Use -ffunction-sections in libbootstrap.a

So that the linker discards unused functions too. Some of our .cpp are
way too big

Change-Id: I1a2685be6a5e7fd3cf34f18d545483c63c2343dd
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

src/tools/bootstrap/bootstrap.pro

@@ -114,6 +114,7 @@ macx: {
               ../../corelib/kernel/qcore_mac.cpp
    LIBS += -framework CoreServices
 }
+*-g++*: QMAKE_CXXFLAGS += -ffunction-sections
 
 if(contains(QT_CONFIG, zlib)|cross_compile):include(../../3rdparty/zlib.pri)
 else:include(../../3rdparty/zlib_dependency.pri)