x86: ia32_signal: introduce {get|set}_user_seg()

Impact: cleanup

Introduce {get|set}_user_seg() and loadsegment_xx() macros to make code clean.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>