[libc++] Update supported system versions

Reviewed By: ldionne, #libc, emaste, daltenty

Spies: fsb4000, daltenty, alvinhochun, goncharov, phosek, MaskRay, dalias, q66, thesamesam, emaste, libcxx-commits, arphaman, mstorsjo, #libc_vendors

Differential Revision: https://reviews.llvm.org/D140343