Revert "Use PRIVATE_POST_INSTALL_CMD to simplify executable_prefer_symlink.mk"

This breaks our build-art-host and build-art-target. I think we need to rewrite those rules as well.

This reverts commit 29f0a487.

Change-Id: I2e6674e01a2042e9d53e28ecd3e6bdf76fa806a3