x86: switch to using asm-generic for seccomp.h

Switch to using the newly created asm-generic/seccomp.h for the seccomp
strict mode syscall definitions. The obsolete sigreturn syscall override
is retained in 32-bit mode, and the ia32 syscall overrides are used in
the compat case. Remaining definitions were identical.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>