Move std.c to arch to avoid conflicts with libc for tests.

Change-Id: I0fc966962413ee12f84058d702e7396e38460f27
diff --git a/src/layout.c b/src/layout.c
index fe11c91..403d2d4 100644
--- a/src/layout.c
+++ b/src/layout.c
@@ -16,7 +16,7 @@
 
 #include "hf/layout.h"
 
-#include "hf/std.h"
+#include "hf/arch/std.h"
 
 /**
  * Get the address the .text section begins at.