CI: Add cross-compile tests for some Linux targets

[tatsuya6502]

Add a CI (GitHub Actions) using rust-embedded/cross. It does the followings:

1. Cross-compile Moka to a specified Linux target.
2. Run cross test(similar to cargo test but without integration tests) using QEMU user space emulation.

Currently, the following targets are tested:

• aarch64-unknown-linux-musl
• i686-unknown-linux-musl
• armv7-unknown-linux-musleabihf
• armv5te-unknown-linux-musleabi
• mips-unknown-linux-musl
• mipsel-unknown-linux-musl

There is one concern though. The README of cross says:

cross test runs units tests sequentially because QEMU gets upset when you spawn multiple threads. This means that, if one of your unit tests spawns threads, then it's more likely to fail or, worst, never terminate.

Moka does spawn threads but it seems QEMU is working fine. So it is OK for now. If such a problem happens in the future, we will have to disable the cross test step.

[tatsuya6502]

Merging.

[tatsuya6502]

There is one concern though. The README of cross says:

cross test runs units tests sequentially because QEMU gets upset when you spawn multiple threads. This means that, if one of your unit tests spawns threads, then it's more likely to fail or, worst, never terminate.

Moka does spawn threads but it seems QEMU is working fine. So it is OK for now. If such a problem happens in the future, we will have to disable the cross test step.

I realized the README was written in December 2016. So the QEMU's user-space emulatiton may have been improved since then. This could explain why cross test is working for me.

• cross-rs/cross@531a8b8#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R68-R70