Make the INTO instruction operational in 32bit mode.

Having the IDT entry specify ring 0 DPL caused delivery of #GP instead
of #OF.

The instruction is not valid in 64bit mode, which probably explains
why the IDT entry for #OF was initially set this way. It is
interesting to note that the BOUND instruction works with the IDT #BR
entry DPL 0, most likely CPU considers #BR from BOUND as generated by
a machine, not user.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week