Rename plat/linux/libsys/errno.s to plat/linux386/libsys/trapno.s and
stop building it for linux68k and linuxppc. It defines symbols for
mach/i386/libem.
In syscalls.h, the numbers after 165 are only for i386, so hide them
from 68k, ppc. These numbers are unused, because the system calls now
in libsys use the lower numbers.
Also teach the build system that libsys depends on the internal
headers in plat/linux/libsys/*.h
