handle cross-compilation test failure more gracefully

handle cross-compilation test failure more gracefully

The current cross-compilation test module fails every test if an
appropriate cross standard library is not found.  This behavior is
unhelpful, because the failures are verbose, unexpected, and there's no
obvious way to make them go away.

Instead, it would be better to check once before all the tests that an
appropriate cross-compilation setup is available.  Once that check has
been done, a single test can fail with an appropriate `panic!` message
while the other tests silently pass.  The `panic!` message can inform
the user about their options, either setting an appropriate environment
variable, or using rustup to install the necessary cross standard
library.

Assuming the user has rustup installed, the single failure now looks
something like:

```
	thread 'plugin_deps' panicked at 'Cannot cross compile to i686-unknown-linux-gnu.

This failure can be safely ignored. If you would prefer to not see this
failure, you can set the environment variable CFG_DISABLE_CROSS_TESTS to "1".

Alternatively, you can install the necessary libraries for cross-compilation with

    rustup toolchain install stable-i686-unknown-linux-gnu

You may need to install runtime libraries for your Linux distribution as well.
', tests/cross-compile.rs:87
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```

Fixes #3086.