[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/ELF/ObjectFileELF.cpp

@@ -51,6 +51,8 @@ using namespace lldb_private;
 using namespace elf;
 using namespace llvm::ELF;
 
+LLDB_PLUGIN(ObjectFileELF);
+
 namespace {
 
 // ELF note owner definitions