make target-run compiled artifacts per-arch

[graydon]

once in sysroot, librustrt.so should be built as stageN/lib/$arch/librustrt.so and recompiled for every target in the targets list for the current compiler. rustc and librustllvm should probably only be compiled for every host in the host list. Or, if we're lucky, we only have one host.

[espindola]

librustrt is written in c++. This would require a cross compiler to implement. Why do we need this?

[brson]

Can this be delayed until after the 0.1 milestone?

[graydon]

Only requires a cross compiler when we support host != target. Until then it's just a question of directory organization. I agree we can defer host != target until after first milestone. I'd like to get the directory organization right though.

[graydon]

Doing this in steps, also combining it with a slight reorg of built dirs into stageN/{bin,lib}/ such that it has the right shape to be installed into /usr/ with the RPATH-ing that has to happen in #441.

First step is to teach configure and the makefiles and compiler to handle --target args, defaulting to host triple, add library search path for $sysroot/lib/rustc/$target, and make $stageN/lib/rustc/$target/ libs build along with the usual ones.

Once all new stuff is building properly, snapshot and start purging the old stuff from the make system.

[brson]

Done.