Define Android API level needed for AArch64.

build.rs

@@ -593,7 +593,12 @@ fn cc(file: &Path, ext: &str, target: &Target, warnings_are_errors: bool,
         // Define __ANDROID_API__ to the Android API level we want.
         // Needed for Android NDK Unified Headers, see:
         // https://android.googlesource.com/platform/ndk/+/master/docs/UnifiedHeaders.md#Supporting-Unified-Headers-in-Your-Build-System
-        let _ = c.define("__ANDROID_API__", Some("18"));
+        if target.arch() == "aarch64" {
+            // Minimum API level where AArch64 is available is 21.
+            let _ = c.define("__ANDROID_API__", Some("21"));
+        } else {
+            let _ = c.define("__ANDROID_API__", Some("18"));
+        }
     }
 
     let mut c = c.get_compiler().to_command();