Multi-target build

[Mygod]

Describe the problem you are trying to solve

When building rust code for an Android app, it is common to build for multiple targets, e.g. (ARM, x86) ⊗ (32 bits, 64 bits). While cargo build is already parallel in some sense, LTO (or last step compilation in general) is single threaded and can take a long time to run. It makes sense to use up the CPU resources for building other targets.

Describe the solution you'd like

Multi-target build with --target=armv7-linux-androideabi --target=aarch64-linux-android ....

Notes

See also plugin: https://github.com/mozilla/rust-android-gradle

[alexcrichton]

Thanks for the report! This is actually a refactoring I've wanted to do for some time now, so I've mustered up the motivation and pushed up #8167 to implement this.

[ehuss]

Closing in favor of tracking issue #8176.