[libc++] Disable failing C11 feature tests for <cfloat> and <float.h>

Summary:
Those tests are breaking the test bots. A Bugzilla has been filed to
make sure those tests are re-enabled: https://bugs.llvm.org/show_bug.cgi?id=38572

Reviewers: mclow.lists, EricWF

Subscribers: krytarowski, christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50748

llvm-svn: 339742