marked semihosting_halt function unsafe

Description

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[ ] Fixed all cargo clippy warnings.
[ ] Applied rustfmt. https://github.com/nuta/kerla/blob/e1ddf73614a7bc5a400e9739b4f3a20af2ed045e/runtime/x64/semihosting.rs#L10-L14 hello, if a function's entire body is unsafe, the function is itself unsafe and should be marked appropriately. Marking them unsafe also means that callers must make sure they know what they're doing.