These tests fail, in general, for cross-compilation, because they require
the rustc crates to exist for the target, and they don't. We can't compile
them for the target unless we also compile LLVM for the target (we don't).
Android is a subset of cross-compilation.
The other fulldeps tests, on the other hand, work fine for
cross-compilation, and in fact, are verifying that rustc correctly searches
for a host plugin crate, not a target plugin crate.
