Revert "Remove incorrect guard check for Android"

[hjyamauchi]

Reverts #5171

This makes it at least buildable on Android.

[hjyamauchi]

@swift-ci please test

[finagolfin]

This is clearly incorrect, as explained in detail in my subsequent comments in #5171.

Since you're having problems dealing with the nullability annotations in the latest Android NDKs, which I noted I haven't upgraded these posix_spawn APIs to yet in my own Android SDK, what may work is to update to NDK 27 and move this Android guard check down a line after the posix_spawnattr_init(), to unwrap the optional in Swift only after you know spawnAttrs has been initialized. Of course, the only way to test that, if it compiles, is to actually run it too, which it appears you haven't been doing so far.

I suggest you experiment with such ways to fix this code with the latest NDKs, rather than simply sticking with whatever temporary compilation-only hack @hyp put in earlier.

[hjyamauchi]

Saleem has this: #5179 for moving the guard code