[libc++] Add availability markup for std::to_chars on Apple platforms

Summary:
Otherwise, one gets link errors when trying to back-deploy to older platforms.

rdar://problem/57854364

Reviewers: lichray, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D74626