[libc++] Recategorize additional instantiations in the dylib as availability macros

Adding additional instantiations to the dylib isn't actually an ABI break as long as programs targeting an older dylib don't start to depend on them. Making additional instantiations a matter of availability allows us to add them without an ABI break.

Reviewed By: #libc, ldionne, Mordante

Spies: arichardson, ldionne, Mordante, libcxx-commits

Differential Revision: https://reviews.llvm.org/D154796