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/x86_64/kernel/entry.S b/arch/x86_64/kernel/entry.S
index be51dbe..3620508 100644
--- a/arch/x86_64/kernel/entry.S
+++ b/arch/x86_64/kernel/entry.S
@@ -36,7 +36,7 @@
 #include <asm/errno.h>
 #include <asm/dwarf2.h>
 #include <asm/calling.h>
-#include <asm/offset.h>
+#include <asm/asm-offsets.h>
 #include <asm/msr.h>
 #include <asm/unistd.h>
 #include <asm/thread_info.h>