ia64/signal: switch to generic struct siginfo

... at a cost of added small ifdef __ia64__ in asm-generic siginfo.h,
that is.

-- EWB Corrected the comment on _flags to reflect the move

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>