Cross-building from M1 to x86 may not work

[gmarkall]

(First observed in the review of #841)

This hasn't hit us because we're not cross-building, but I wonder if cross-compiling from M1 to x86 won't work in the conda package builds, because the target is selected based on the build platform and not the target platform:

https://github.com/numba/llvmlite/blob/409cd6d37fe1ba76438aa1a3a9c6a8adcd0ddb9d/conda-recipes/llvmdev/build.sh#L12-L16