x86-64: Use symbolics instead of raw numbers in entry_64.S

... making the code a little less fragile.

Also use pushq_cfi instead of raw CFI annotations in two more
places, and add two missing annotations after stack pointer
adjustments which got modified here anyway.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Alexander van Heukelum <heukelum@fastmail.fm>
LKML-Reference: <4C7FBACF0200007800013F6A@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>