Add ElfWriter::GetStream().

This will be used for writing the OatHeader which is
currently oddly written through the .text stream.

Also move the error delaying output stream out of the
ElfBuilder<> to its own file and move all output stream
files to compiler/linker/.

Change-Id: I00db4e33ed80ac4757ec459946c7b5ae014a3a2e
diff --git a/compiler/oat_writer.cc b/compiler/oat_writer.cc
index a6a49f9..0087a0d 100644
--- a/compiler/oat_writer.cc
+++ b/compiler/oat_writer.cc
@@ -36,6 +36,7 @@
 #include "gc/space/space.h"
 #include "handle_scope-inl.h"
 #include "image_writer.h"
+#include "linker/output_stream.h"
 #include "linker/relative_patcher.h"
 #include "mirror/array.h"
 #include "mirror/class_loader.h"
@@ -43,7 +44,6 @@
 #include "mirror/object-inl.h"
 #include "oat_quick_method_header.h"
 #include "os.h"
-#include "output_stream.h"
 #include "safe_map.h"
 #include "scoped_thread_state_change.h"
 #include "type_lookup_table.h"