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/oatdump/oatdump.cc b/oatdump/oatdump.cc
index d20f7d5..a99bb00 100644
--- a/oatdump/oatdump.cc
+++ b/oatdump/oatdump.cc
@@ -42,6 +42,8 @@
 #include "gc/space/space-inl.h"
 #include "image.h"
 #include "indenter.h"
+#include "linker/buffered_output_stream.h"
+#include "linker/file_output_stream.h"
 #include "mapping_table.h"
 #include "mirror/array-inl.h"
 #include "mirror/class-inl.h"
@@ -52,7 +54,6 @@
 #include "oat_file-inl.h"
 #include "oat_file_manager.h"
 #include "os.h"
-#include "output_stream.h"
 #include "safe_map.h"
 #include "scoped_thread_state_change.h"
 #include "stack_map.h"