x86-64, reboot: Allow reboot=bios and reboot-cpu override on x86-64

With the revamped realmode trampoline code, it is trivial to extend
support for reboot=bios to x86-64.  Furthermore, while we are at it,
remove the restriction that only we can only override the reboot CPU
on 32 bits.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/n/tip-jopx7y6g6dbcx4tpal8q0jlr@git.kernel.org

arch/x86/include/asm/realmode.h

@@ -21,8 +21,9 @@ struct real_mode_header {
 	u32	wakeup_header;
 #endif
 	/* APM/BIOS reboot */
-#ifdef CONFIG_X86_32
 	u32	machine_real_restart_asm;
+#ifdef CONFIG_X86_64
+	u32	machine_real_restart_seg;
 #endif
 };