Do not run Fiber unit test on certain systems

Systems which do use the C backend but the C library returns ENOSYS at runtime. (This happens on ARM with glibc < 2.17 for example).

We do not want to abort all testing in this case. We also don't want the static ctor to assert in this case whenever someone wants to use core.thread.

Note: This is part of a set of changes which are required to get test suite & unit tests passing on ARM GDC. All necessary changes for GDC are here for reference:
https://github.com/jpf91/GDC/commits/arm-old

Once these pull requests are merged upstream I'll merge them into GDC as well.