[libc++] Bumping minimal deployment target for building the dylib / static library on macOS

Building libc++ and libc++abi for Apple platforms will soon require targeting macOS 10.13 and later. This does NOT impact users building their code using libc++ headers and targeting older OSes, where we still support back to macOS 10.9 as documented.

This is only relevant for vendors building the libc++ shared library and for folks statically linking libc++ into an application that back-deploys to older OSes. As far as we can tell, the only remaining client that needed older than macOS 10.13 was Chrome, and they have now moved to requiring macOS 10.13, hence the bump on our side.

Please target additional discussion in ⚙ D145012 [libc++] Clean up old macOS back-deployment workarounds, it will ensure that all interested parties see it. I plan to land that patch on March 10th.