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/dwarf/dwarf_test.h b/compiler/dwarf/dwarf_test.h
index 5464ed9..c3a3ca9 100644
--- a/compiler/dwarf/dwarf_test.h
+++ b/compiler/dwarf/dwarf_test.h
@@ -29,6 +29,7 @@
 #include "common_runtime_test.h"
 #include "elf_builder.h"
 #include "gtest/gtest.h"
+#include "linker/file_output_stream.h"
 #include "os.h"
 
 namespace art {