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_test.cc b/compiler/oat_test.cc
index c305b12..b8610d0 100644
--- a/compiler/oat_test.cc
+++ b/compiler/oat_test.cc
@@ -31,13 +31,13 @@
 #include "elf_writer.h"
 #include "elf_writer_quick.h"
 #include "entrypoints/quick/quick_entrypoints.h"
+#include "linker/vector_output_stream.h"
 #include "mirror/class-inl.h"
 #include "mirror/object_array-inl.h"
 #include "mirror/object-inl.h"
 #include "oat_file-inl.h"
 #include "oat_writer.h"
 #include "scoped_thread_state_change.h"
-#include "vector_output_stream.h"
 
 namespace art {