x86: Improve cmpxchg8b_emu.S

- don't include unneeded headers
- drop redundant entry point label
- complete unwind annotations
- use .L prefix on local labels to not clutter the symbol table

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/5422917E0200007800038081@mail.emea.novell.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>