x86: fix sigcontext.h user export

Jakub Jelinek reported that some user-space code that relies on
kernel headers has built dependency on the sigcontext->eip/rip
register names - which have been unified in commit:

  commit 742fa54a


  Author: H. Peter Anvin <hpa@zytor.com>
  Date:   Wed Jan 30 13:30:56 2008 +0100

      x86: use generic register names in struct sigcontext

so give the old layout to user-space. This is not particularly
pretty, but it's an ABI so there's no danger of the two definitions
getting out of sync.

Reported-by: Jakub Jelinek <jakub@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>