enable async feature will build fail with musl env

[CaptainVincent]

The default feature will enable async.

However, many functions or structures in executor are used in the fiber.rs but are ignored with #[cfg(all(target_os = "linux", not(target_env = "musl")))] cause build fail.

Is this the expected behavior? and is there any standard approach for handling this? Currently, I can only always disable async in runwasi. (they will build with musl and not musl both)