Fix statx feature test for Android

statx syscall was introduced in Android-30.
Compiling for Android<30 should fail, yet it doesn't, which results in false positive for a feature that is actually not available.
Would guess that this happens because of the same name for both the function and the struct, which is not #ifdef'ed for legacy Android.

Current check "host_system != 'android'" doesn't guard the feature test against android,
because host_system is never just android, it's android-$abi, like android-x86_64.
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