cpufeatures: Use getauxval for aarch64 Android as well

[jrose-signal]

libc::getauxval was added for Android in rust-lang/libc#1987, and is available on all aarch64 Android devices, so we should be able to detect features there as well.

[tarcieri]

Mind submitting a PR for this? I've never used Rust on Android before

[jrose-signal]

Now I have to be the one to double-check that its results are the same as Linux. :-) Hang on…