x86, espfix: Move espfix definitions into a separate header file

Sparse warns that the percpu variables aren't declared before they are
defined.  Rather than hacking around it, move espfix definitions into
a proper header file.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

diff --cc arch/x86/include/asm/espfix.h
Simple merge

diff --cc arch/x86/include/asm/setup.h
Simple merge

diff --cc arch/x86/kernel/espfix_64.c
Simple merge