fix: Flip signs in Time.h's Apple version checks

[amgleitman]

When trying to bring react-native-macos up to date with 0.66, I was running into an issue getting RNTester to compile due to an error regarding redefining clockid_t. Other people have been seeing similar issues as per these search results.

The history behind this appears to be as follows:

Several declarations in <time.h> were not available on Apple platforms until macOS 10.12 and iOS 10, which is why Folly needs to check the minimum version and set FOLLY_HAVE_CLOCK_GETTIME as needed. The problem is, the current logic as it stands right now is to set FOLLY_HAVE_CLOCK_GETTIME = 1 (which implies that we don't need to declare them ourselves as the OS provides them for us) if...

• ...we're building for macOS, and the minimum required version is less than 10.12, or...
• ...we're building for iOS, and the minimum required version is less than 10.

However, this doesn't make any sense. This is saying that we don't need to declare these missing APIs if we could be compiling Folly for use on an older version (i.e., macOS 10.11/iOS 9 or earlier), which is totally wrong! Instead, we ought to be checking if the versions are at least macOS 10.12 or iOS 10.

React Native currently works around this by eliminating the minimum version check entirely with this PR, which is certainly a valid local fix (the minimum iOS version for React Native apps is currently iOS 11), but doesn't solve the problem at its core. This PR does solve the problem.

I have not tested building this with a minimum version below the above thresholds for use on a modern version of macOS/iOS, but considering the discussion in #1545, I think we should be safe to ignore these older versions from now on.

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[Saadnajmi]

@kelset I remember there was a lot of issues around the post_install_script phase on iOS around the RN Core 0.66 release. I think this is related, and a more long term fix that may help with removing the need for that phase.

[Saadnajmi]

New year, new ping! Anything we can do to move this PR forward? I'm somewhat optimistic it'll help remove some of the "hacks" were needed for RN 0.66 =)

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