kbuild: mips use generic asm-offsets.h support

Removed obsolete stuff from arch makefile.
mips had a special rule for generating asm-offsets.h so preserved it
using an architecture specific hook in top-level Kbuild file.
Renamed .h file to asm-offsets.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/mips/lib/strnlen_user.S b/arch/mips/lib/strnlen_user.S
index 6e7a8ee..c0ea151 100644
--- a/arch/mips/lib/strnlen_user.S
+++ b/arch/mips/lib/strnlen_user.S
@@ -7,7 +7,7 @@
  * Copyright (c) 1999 Silicon Graphics, Inc.
  */
 #include <asm/asm.h>
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 #include <asm/regdef.h>
 
 #define EX(insn,reg,addr,handler)			\