kbuild: arm - use generic asm-offsets.h support

Delete obsoleted stuff from arch Makefile and rename
constants.h to asm-offsets.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
index 1155cf0..5396263 100644
--- a/arch/arm/kernel/head.S
+++ b/arch/arm/kernel/head.S
@@ -20,7 +20,7 @@
 #include <asm/mach-types.h>
 #include <asm/procinfo.h>
 #include <asm/ptrace.h>
-#include <asm/constants.h>
+#include <asm/asm-offsets.h>
 #include <asm/thread_info.h>
 #include <asm/system.h>