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

commit e1fe9ed8d2a4937510d0d60e20705035c2609aea upstream.

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.

Change-Id: I471e41c7af1b51c58fa0aa7828080261e4a57fd5