kbuild: alpha,x86_64 use generic asm-offsets.h support

Delete obsolete stuff from arch makefiles
Rename .h file to asm-offsets.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/alpha/kernel/head.S b/arch/alpha/kernel/head.S
index 4ca2e40..0905721 100644
--- a/arch/alpha/kernel/head.S
+++ b/arch/alpha/kernel/head.S
@@ -9,7 +9,7 @@
 
 #include <linux/config.h>
 #include <asm/system.h>
-#include <asm/asm_offsets.h>
+#include <asm/asm-offsets.h>
 
 .globl swapper_pg_dir
 .globl _stext