Do not define template specialization __libcpp_is_floating_point<__fp16>

if the compiler is not clang.

gcc doesn't allow using __fp16 on non-ARM targets.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@333108 91177308-0d34-0410-b5e6-96231b3b80d8