Add getauxval for Android

[jack-signal]

getauxval is useful for detecting CPU specific features such as availability of the ARMv8 AES instructions.

Currently in this crate, getauxval is available on Linux but not on Android. This function was added to Android in API Level 18 (https://developer.android.com/ndk/guides/cpu-features). So it might be a problem for someone building to an older API level. However on aarch64 and x86_64 the earliest API Level is 21 and thus for these architectures getauxval is always available.

Happy to contribute a patch, just wanted to check if enabling getauxval only for aarch64-linux-android and x86_64-linux-android was ok or if you had suggestions on how to handle the possibility of it not being included when built for an old Android.

[JohnTitor]

Hey, thanks for the issue!

Happy to contribute a patch, just wanted to check if enabling getauxval only for aarch64-linux-android and x86_64-linux-android was ok or if you had suggestions on how to handle the possibility of it not being included when built for an old Android.

It's totally okay to add it for these two targets.