Detect CPU features with Linux methods on Android for non-Intel CPUs.

[gendx]

After digging into why the is_aarch64_feature_detected! macro didn't work as I expected on my Android device (only asimd, fp, neon were detected by the macro), I came to this line that dispatches the detection depending on the OS.

My understanding is that Android should behave like Linux w.r.t. CPU feature detection on ARM - at least on the devices I've tested with the various methods (getauxval, checking /proc/cpuinfo) worked fine. However, Rust defines a distinct target_os for Android (https://github.com/rust-lang/rust/blob/1.65.0/compiler/rustc_target/src/spec/android_base.rs#L5), so the filtering by target_os = "linux" doesn't work (even though Android is arguably a flavor of Linux).

I've tested this patch manually on a stage 1 compiler and it worked on physical devices (Samsung Galaxy S7 and Samsung Galaxy S20).

I wonder if the feature = "libc" is relevant on Android?

I didn't try the CI script but I guess it's not so relevant at the moment since Android CI was removed (#1346)? I wonder if this PR makes a CI for Android more relevant - although it may be that under the hood qemu doesn't implement as many CPU features as physical devices do?

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) soon.

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[Amanieu]

Regarding CI, there are currently some issues on the Rust side that need to be resolved (rust-lang/rust#103673). Unfortunately I currently don't have the time to look into these issues.

[thomcc]

I don't think we use the "libc" feature in the stdlib, do we?

[Amanieu]

We do use it, otherwise feature detection wouldn't work on non-x86 platforms.

[thomcc]

We do use it, otherwise feature detection wouldn't work on non-x86 platforms.

I believe it doesn't?

• https://github.com/rust-lang/rust/blob/master/library/std/Cargo.toml#LL23
• https://github.com/rust-lang/stdarch/blob/master/crates/std_detect/Cargo.toml#L41-L45

I don't see how the libc feature would be enabled. I thought this was intentional (Last time I looked, some of that code seemed to be a bit sketchy, but looking now it's probably fine?)

[Amanieu]

It's detected via these features:

• https://github.com/rust-lang/rust/blob/master/library/std/Cargo.toml#L73

Which are enabled by default here:

• https://github.com/rust-lang/rust/blob/master/library/test/Cargo.toml#L23