[PATCH] x86-64: miscellaneous entry.S adjustments

This patch:
- makes ret_from_sys_call no longer global (all external users were
  previously switched to use int_ret_from_sys_call)
- adjusts placement of a CFI_{REMEMBER,RESTORE}_STATE pair to better
  fit logic flow
- eliminates an unnecessary pair of CFI_{REMEMBER,RESTORE}_STATE
- glues together function- and unwinder-wise the previously separate
  system_call and int_ret_from_sys_call function fragments

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>