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:

llvmlite/conda-recipes/llvmdev/build.sh

Lines 12 to 16 in 409cd6d

	if [[ $build_platform == osx-arm64 ]]; then
	DARWIN_TARGET=arm64-apple-darwin20.0.0
	else
	DARWIN_TARGET=x86_64-apple-darwin13.4.0
	fi