seccomp: Support IP-based syscall filtering

Syscalls coming from code that is maintained as part of the kernel
don't necessarily need to follow the generic syscall ABI.

To enable filters to disriminate userspace syscall sites that a
controlled by the kernel, add instruction pointer bounds to struct
seccomp_data.

Filters can check whether the userspace instruction pointer falls
within these bounds when a syscall is attempted, and can apply
whatever policy they like.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>