[Feature Request] Support all tier 2 targets and check in CI

[SpriteOvO]

As of this issue being opened, we have received 2 reports (#36 #82) of the compile error due to platform-specific code.

Rust has a target tier policy, in Rust doc, the definition of tier 2 is

Tier 2 targets can be thought of as "guaranteed to build". The Rust project builds official binary releases of the standard library (or, in some cases, only the core library) for each tier 2 target, and automated builds ensure that each tier 2 target can be used as build target after each change. Automated tests are not always run so it's not guaranteed to produce a working build, but tier 2 targets often work to quite a good degree and patches are always welcome!

IMO, tier 2 covers most common targets. It would be nice if we apply the same "guaranteed to build" policy for spdlog-rs, so that we can support as many as possible targets in advance, without waiting for bug reports from users.

TODO

• [ ] Add a CI to check all the compilation of all tier 2 targets.
  • How to get a list of all tier 2 targets?
    • rustc +nightly --print all-target-specs-json -Z unstable-options
• [ ] Make platform-specific code to support these all platforms.