[lldb/Plugins] Use external functions to (de)initialize plugins

This is a step towards making the initialize and terminate calls be
generated by CMake, which in turn is towards making it possible to
disable plugins at configuration time.

Differential revision: https://reviews.llvm.org/D74245

lldb/source/Plugins/OperatingSystem/Python/OperatingSystemPython.cpp

@@ -39,6 +39,8 @@
 using namespace lldb;
 using namespace lldb_private;
 
+LLDB_PLUGIN(OperatingSystemPython);
+
 void OperatingSystemPython::Initialize() {
   PluginManager::RegisterPlugin(GetPluginNameStatic(),
                                 GetPluginDescriptionStatic(), CreateInstance,