[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/PECOFF/ObjectFilePECOFF.cpp

@@ -41,6 +41,8 @@
 using namespace lldb;
 using namespace lldb_private;
 
+LLDB_PLUGIN(ObjectFilePECOFF);
+
 struct CVInfoPdb70 {
   // 16-byte GUID
   struct _Guid {