x86-64: add symbolic constants for the boot segment selectors

Add symbolic constants for the segment selectors/GDT slots used by
the setup code, for consistency with i386.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/asm-x86_64/segment.h

@@ -3,6 +3,14 @@
 
 #include <asm/cache.h>
 
+/* Simple and small GDT entries for booting only */
+
+#define GDT_ENTRY_BOOT_CS		2
+#define __BOOT_CS	(GDT_ENTRY_BOOT_CS * 8)
+
+#define GDT_ENTRY_BOOT_DS		(GDT_ENTRY_BOOT_CS + 1)
+#define __BOOT_DS	(GDT_ENTRY_BOOT_DS * 8)
+
 #define __KERNEL_CS	0x10
 #define __KERNEL_DS	0x18