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/realmode/rm/header.S

@@ -6,6 +6,7 @@
 
 #include <linux/linkage.h>
 #include <asm/page_types.h>
+#include <asm/segment.h>
 
 #include "realmode.h"
 	
@@ -28,8 +29,9 @@ GLOBAL(real_mode_header)
 	.long	pa_wakeup_header
 #endif
 	/* APM/BIOS reboot */
-#ifdef CONFIG_X86_32
 	.long	pa_machine_real_restart_asm
+#ifdef CONFIG_X86_64
+	.long	__KERNEL32_CS
 #endif
 END(real_mode_header)