Fix building libc++ with the macOS 10.13 SDK with -mmacosx-version-min=10.12 or lower.

The 10.13 SDK always defines utimensat() (with an availability(macosx=10.13) annotation)
and unconditionally defines UTIME_OMIT, so use the compile-time availability macros
on Apple platforms instead.

For people statically linking libc++, it might make sense to also provide an opt-in
option for using __builtin_available() to dynamically check for the OS version,
but for now let's do the smallest thing needed to unbreak the build.

Based on a patch by Eric Fiselier <eric@efcs.ca>: https://reviews.llvm.org/D34249
Fixes PR33469.

llvm-svn: 324385