[Darwin][compiler-rt] kern.osproductversion unavailable pre macOS 10.13

Add a fallback for `sysctl kern.osproductversion` for XNU 17 (macOS
10.13) and below, which do not provide this property.

Unfortunately, this means we have to take the detour via Darwin kernel
version again (at least for the fallback).

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D84892