The feature isn't defined anywhere, so it's effectively dead.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
-//
-// UNSUPPORTED: newlib
// <cfenv>
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
-//
-// XFAIL: newlib
// <fenv.h>
//
// XFAIL: suse-linux-enterprise-server-11
// XFAIL: darwin
-// XFAIL: newlib
// XFAIL: netbsd
// <uchar.h>
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
-//
-// UNSUPPORTED: newlib
// <cfenv>