x86: signal: rename COPY_SEG_STRICT to COPY_SEG_CPL3

Impact: cleanup

Rename macro COPY_SEG_STRICT to COPY_SEG_CPL3, as suggested by hpa.

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