panic-semihosting docs are empty

[jonas-schievink]

https://docs.rs/panic-semihosting/0.5.4/panic_semihosting/ is empty, despite there being documentation on the crate.

Caused by #![cfg(all(target_arch = "arm", target_os = "none"))].

The fix is to tell docs.rs to use an embedded ARM target (see https://docs.rs/about/metadata).