x86: reboot_{32|64}.c unification

reboot_{32|64}.c unification patch.

This patch unifies the code from the reboot_32.c and reboot_64.c files.

It has been tested in computers with X86_32 and X86_64 kernels and it
looks like all reboot modes work fine (EFI restart system hasn't been
tested yet).

Probably I made some mistakes (like I usually do) so I hope
we can identify and fix them soon.

Signed-off-by: Miguel Boton <mboton@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>