During cross build, the same obj directory is used for target and host libunwind compilation

[janvorli]

This leads to two rebuilds of the libunwind even without any changes in its sources.

[janvorli]

cc: @jkoritzinsky

[janvorli]

The root of the problem seems to be in the fact that the CLR_ARTIFACTS_OBJ_DIR doesn't contain the os/arch/flavor triplet. But it is not as simple as adding those to this path.