[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/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -56,6 +56,8 @@ using namespace lldb_private;
 using namespace lldb_private::python;
 using llvm::Expected;
 
+LLDB_PLUGIN(ScriptInterpreterPython);
+
 // Defined in the SWIG source file
 #if PY_MAJOR_VERSION >= 3
 extern "C" PyObject *PyInit__lldb(void);