Skip testing targets that don't ship libstd

[Nemo157]

When you have weirder targets like thumbv6m-none-eabi installed in your current toolchain then compiletest crashes because it doesn't know what os or architecture the target is:

> cargo test
[...]
## Running run-pass tests in tests/run-pass against miri for target thumbv6m-none-eabi
thread 'main' panicked at 'Cannot determine OS from triple', /Users/nemo157/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/compiletest_rs-0.3.17/src/util.rs:65:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: test failed, to rerun pass '--test compiletest'

Then, after hacking compiletest to run anyway, the tests fail because there is no std distributed with the target.

This change will fix the second issue by not attempting to run the tests if there is no libstd distributed with the target, which has a side-effect of fixing the first issue for at least embedded targets. There may be other targets that compiletest has an issue with, but do distribute libstd, but I can't see any easy way to deal with them.

[RalfJung]

Looks good, thanks! Will merge once CI is happy.