Mirror for each build_arch

[ryandesign (Ryan Carsten Schmidt)]

The number of ports that want to download different distfiles for different values of build_arch seems to be increasing and mpbb doesn't currently support that when mirroring. It only supports mirroring different distfiles based on os.arch but that doesn't help if you need a different distfile for i386 than for x86_64. ghc-bootstrap is an example of such a port. I've mirrored its i386 distfile manually but maybe mpbb could handle it automatically.

Mirroring used to take a long time but I think the rewrite as mirror-multi.tcl has made it faster so that it might be feasible to mirror for each build_arch now? I'm envisioning that mirroring for darwin 8 and 9 on powerpc would run twice, once setting build_arch and configure.build_arch to ppc and once to ppc64; and mirroring for darwin 8, 9, and 10 on i386 would run twice, once for i386 and once for x86_64. For darwin 11 and later on each OS arch, only the one mirroring run would be needed as before, but with the addition of setting the build arch for good measure.