android: stdlib stack overflow handlers are not installed

When updating the CI to newer NDK/APIs, I enabled aarch64-linux-android testing, and found that tests/ui/abi/stack-probes.rs was failing. Luckily, the stack probes themselves are working (the expected SIGSEGV is being triggered), but as the signal handler is not installed, the test fails and the user wouldn't receive a helpful error + backtrace in the real world.

The registration is currently in an odd state where the implementation is enabled for Linux but not Android, and yet contains Android-specific cfgs inside.

Enabling it (by adding "android" to both the mod cfg blocks) works on aarch64, but on arm, it fails because getauxval is missing from libc.

The route to fixing this:

1. Add the getauxval declaration to libc android 32-bit (it's already in 64-bit). The original argument for not including it is that it was added in Android API 18. Since we're bumping to 21+ as the minimum API, that should not be an issue.
2. Add android to the registration block
3. Remove the ignore-android I'm about to add from tests/ui/abi/stack-probes.rs and tests/ui/abi/stack-probes-lto.rs and check that they still work.

cc @chriswailes Perhaps you'd like to take this one?

Start with library/std/src/sys/pal/unix/stack_overflow.rs and the 32-bit Android module in libc, then inspect tests/ui/abi/stack-probes.rs and stack-probes-lto.rs. Verify the Android stack-overflow registration and getauxval availability, remove the Android ignores, and run both tests to confirm the handlers work on Android targets.