Omit atomic_{,un}signed_lock_free if unsupported

On targets that have limited atomic support, e.g. ones that define
ATOMIC_*_LOCK_FREE to '1' ("sometimes lock free"), we would end up
referencing yet-undefined __libcpp_{,un}signed_lock_free.

This commit adds a guard to prevent these references for such
targets.

Differential Revision: https://reviews.llvm.org/D118391