fix: use std::ffi::c_char instead of platform specific int

[dekobon]

Proposed changes

This change reverts a previous change that did platform detection and set a type correspondingly to u8 or i8. By using std::ffi::c_char we get a portable way to do the same thing.

This PR fixes a shortcoming with the change in commit: d66f3418d64a4acf18d6b9139db89cd0ec775690

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• [X] I have written my commit messages in the Conventional Commits format.
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[dekobon]

@bavshin-f5 Thank you for the correction regarding the type usage.