Attempt to unbreak filesystem tests on certain linux distros.

On some platforms clock_gettime is in librt, which we don't
link by default when building the tests. However it is required
by the filesystem tests.

This patch introduces a workaround which links librt whenever
the filesystem tests are enabled. The workaround should later
be replaced with a patch that selectively links both libc++fs
and librt only when building filesystem specific tests. However,
the way the test configuration is set up right now, this is
non-trivial.

llvm-svn: 340406