i386 jail on amd64 continued

[xorquewasp at googlemail.com]

Hello.

I appear to have a semi-working i386 tinderbox jail on AMD64
but am getting incorrect results for some ports.

The jail appears to identify itself as ARCH=amd64 and I've
been unable to override this.

This causes ports such as glib to fail to compile as they
contain arch-specific assembler code and the wrong code is
assembled.

Any idea how to fix this?