x86: make sure load_percpu_segment has no stackprotector

load_percpu_segment() is used to set up the per-cpu segment registers,
which are also used for -fstack-protector.  Make sure that the
load_percpu_segment() function doesn't have stackprotector enabled.

[ Impact: allow percpu setup before calling stack-protected functions ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>