I can reproduce with my Android AArch64 SDK by compiling your test file with this command:
./swift/swift-5.3.1-RELEASE-ubuntu20.04/usr/bin/swiftc
-resource-dir swift-android-aarch64-24-sdk/usr/lib/swift
-sdk swift/android-ndk-r21d/toolchains/llvm/prebuilt/linux-x86_64/sysroot/
-tools-directory swift/android-ndk-r21d/toolchains/llvm/prebuilt/linux-x86_64/bin/
stdio.swift -target aarch64-unknown-linux-android21
It's because the ClangImporter doesn't recognize anything other than simple preprocessor defines, I ran into another problem recently when porting swift-nio to Android. I don't think @compnerd's solution will work, as there's currently no built-in way to version Swift code on the Android API level.