Introduce intermediate build target for std.

This gives a place for common functions to be defined.

Change-Id: Ifac38e647ab1042a00d224a58e28a0e6807fa60c
diff --git a/src/layout.c b/src/layout.c
index f74f7c2..e53361b 100644
--- a/src/layout.c
+++ b/src/layout.c
@@ -16,7 +16,7 @@
 
 #include "hf/layout.h"
 
-#include "hf/arch/std.h"
+#include "hf/std.h"
 
 /**
  * Get the address the .text section begins at.