[libc] Fix floating point test failing to build on the GPU

A patch enabled this test which uses that `add_fp_unittest`.
Unfortunately we do not support these on the GPU because it attempts to
link in the floating point utils which are not built supporting
hermetic tests. This was attempted to be fixed in D151123 but that had
to be reverted. For now disable these so the tests pass.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D152742