Invoke `backtrace-rs`' buildscript in `std`'s buildscript for Android targets

[Arc-blroth]

On Android targets, backtrace-rs uses the buildscript to detect if the Android API level, which is needed to enable backtrace symbolication on targets with Android API >= 21. This PR ensures that backtrace-rs's buildscript is invoked as part of the standard library directly including backtrace.

Alongside the patch from rust-lang/cc-rs#705, this should fix backtraces not being symbolicated on Android, though I have only tested this when building from Windows.

Note that this PR will be essentially a no-op as of right now, since the Rust CI builds with a minimum Android API level of 14 (see rust-lang/release-team#9). A custom toolchain build with a higher minimum API level is required to actually get backtraces with symbols as of right now.

A config.toml for building with the latest Android NDK

profile = "library"
changelog-seen = 2


[build]
target = ["x86_64-pc-windows-msvc", "armv7-linux-androideabi"]


[target.armv7-linux-androideabi]
android-ndk = "C:/Users/arcdev/AppData/Local/Android/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/windows-x86_64"
cc = "C:/Users/arcdev/AppData/Local/Android/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/windows-x86_64/bin/armv7a-linux-androideabi21-clang.cmd"
cxx = "C:/Users/arcdev/AppData/Local/Android/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/windows-x86_64/bin/armv7a-linux-androideabi21-clang++.cmd"
linker = "C:/Users/arcdev/AppData/Local/Android/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/windows-x86_64/bin/armv7a-linux-androideabi21-clang++.cmd"
llvm-libunwind = "system"
ar = "C:/msys64/mingw64/bin/ar.exe"

According to the latest Android Platform API Version Distribution, only about 0.4% of Android devices are on Android API version 19 or below, though the latest NDK still supports a lowest API version of 19 (but only for armv7a-linux-androideabi and i686-linux-android and not the other architectures ???).

[rust-highfive]

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

Please see the contribution instructions for more information.

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-log-analyzer]

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
  --> library/std/../backtrace/build.rs:13:9
   |
13 |     let expansion = match cc::Build::new().file("src/android-api.c").try_expand() {
   |
   = help: the trait `Sized` is not implemented for `[u8]`
   = note: all local variables must have a statically known size
   = help: unsized locals are gated as an unstable feature

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[bors]

:umbrella: The latest upstream changes (presumably #101946) made this pull request unmergeable. Please resolve the merge conflicts.