GetMacosAlignedVersion() fails if sysctl is not setup

`GetMacosAlignedVersion()` fails for ASan-ified launchd because the
sanitizer initialization code runs before `sysctl` has been setup by
launchd.  In this situation, `sysctl kern.osproductversion` returns a
non-empty string that does not match our expectations of a
well-formatted version string.

Retrieving the kernel version (via `sysctl kern.osrelease`) still works,
so we can use it to add a fallback for this corner case.

Differential Revision: https://reviews.llvm.org/D94190