[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/ObjectFile/Breakpad/ObjectFileBreakpad.cpp

@@ -16,6 +16,8 @@ using namespace lldb;
 using namespace lldb_private;
 using namespace lldb_private::breakpad;
 
+LLDB_PLUGIN(ObjectFileBreakpad);
+
 namespace {
 struct Header {
   ArchSpec arch;