Change default debugger for OS X from gdb to lldb

The --debuggerengine int argument is casted to the
DebuggerEngineType enum in Qt Creator
(src/plugins/debugger/debuggerconstants). Here, LldbEngineType is defined
as 0x100.

Task-number: QTBUG-41837
Change-Id: I29eca56fc3f548e4b33d944bfd004687c56a12f3
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>