x86: <asm/segment.h>: boot GDT entries are 32/64-independent

The boot GDT entries are common between 32- and 64-bit mode, so move
them to common code instead of having two identical copies.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>