The `std` test straightforwardly can't work without file descriptors;
#99417 tracks moving it out of tests/ui.
`issue-13560.rs` requires the target to support dynamic linking.
`extern-mod-syntax` is interesting. The original test was added to check
if `extern mod` could be parsed correctly and used `extern mod std` and
an import:
138dc3048a (diff-73700e1e851b7a37bc92174635dab726124c82e5bfabbbc45b4a3c2e8e14fadd)
At some point `std::json::Object` was moved out of std to an unstable
rustc-only `extras` crate, and rather than just changing the import it
got changed to use the unstable crate. When `extras` was removed, people
assumed the test was meant to also test rustc_private and changed it to
another unstable crate rather than using something in std.
This changes the test to remove the `rustc_private` import, to allow it
to work properly when cross-compiling.
