Add prctl(2) to compat_linux

Hi all,

I've added support for 2 pieces of the linux prctl(2) syscall. I'm
not sure that what I've got is totally right but it does seem to work
when I attempt to run a program that uses this syscall -- or at least
it doesn't crash the program.

If someone could take a look at my diff here[1], it would be gradly
appreciated!

Thanks,

kev

[1] https://ftp.netbsd.org/pub/NetBSD/misc/ktnb/compat_linux_prctl.diff