android: Set SDK/API level via version-suffxed --target triple

[MarijnS95]

Fixes https://github.com/Traverse-Research/evolve/issues/1058. Our version of rust-mobile#209, see that PR for all details.

[MarijnS95]

This seems to work on small test-apps, but fails on our main app with:

01-07 17:39:59.432  1141  1141 E AndroidRuntime: java.lang.UnsatisfiedLinkError: dlopen failed: TLS symbol "(null)" in dlopened "/data/app/~~cHL8LBKb7eumhNxrtXw63g==/nl.traverse_research.evolve-u9FL5VS6IhAJMmQ9_7K5kg==/lib/arm64/libevolve_android.so" referenced from "/data/app/~~cHL8LBKb7eumhNxrtXw63g==/nl.traverse_research.evolve-u9FL5VS6IhAJMmQ9_7K5kg==/lib/arm64/libevolve_android.so" using IE access model

I did not run into that before testing, perhaps because I upgraded the backtrace crate before? Testing ongoing.

[MarijnS95]

A lot of chasing later, I got this figured out. Wasn't going to settle for a -D__ANDROID_API__={min_sdk_ver} hack.

I couldn't initially reproduce this with cargo-apk because I had a too-old NDK installed (26.3.11579264). After upgrading to 27.2, the issue reproduces there as well.

Searching for it, it's supposedly an intended change where ELF TLS is automatically enabled for SDK level >=29 in upstream LLVM and also in NDK r26b (I was supposedly on r26a): android/ndk#1679. In xbuild, we weren't telling clang what min API version to compile for.

Via golang/go#69109 (comment) we also see that the STATIC_TLS flag is added to our binary. Turns out that's not because of a compiler flag (or lack thereof), but because the rpmalloc crate sets a static TLS model which results in this error: mjansson/rpmalloc#332 / mjansson/rpmalloc#347.

I don't know if we can opt-out of this with a compiler flag, but there are a few things we can do:

1. git reference the linked rpmalloc PR to fix this;
2. Disable rpmalloc for Android (we also disabled it for MacOS due to similar strange issues);
3. Keep not telling clang about our API version, and create a bigger gap with the Android ecosystem by hacking it with -D__ANDROID_API__={min_sdk_ver} instead 🤮

[maxded]

I would be fine with the git reference fix so that we can move on and actually get further into our certification process. Would be good to also share this information with the xbuild issue to get some outside eyes on it (if anyone even does that 👀 ).

Disable rpmalloc for Android (we also disabled it for MacOS due to similar strange issues);

I'm unsure what kind of impact this would have but could be an option as well, I'll leave the decision up to you.

[MarijnS95]

@maxded thanks, that's the way I'm leaning too. Using that commit is quite a divergence so I might just cherry-pick it to an rpmalloc fork.

[MarijnS95]

@maxded note that this repository is public (it is our fork of xbuild) and I can easily link or copy-paste the issue may anyone else observe it.

I'm only opening the PR here and on the upstream side because we have quite a divergent branch where none of the new features have been written/PRd against upstream. I would have to go through everything that was pushed and PR it upstream so that we can get rid of it, or at the very least rebase when all these fixes land upstream.