x86: signal: move macros out from restore_sigcontext()

move macros, COPY, COPY_SEG*, GET_SEG, out from restore_sigcontext().
x86_64: introduce COPY_SEG_STRICT for cs.

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