Handle platforms where c_int is not i32

[robin-nitrokey]

This PR adds a CI job that tests compatibility with an avr target (16-bit) and fixes code that assumes that c_int is always i32.

Replaces:

• https://github.com/trussed-dev/littlefs2/pull/68

One remaining problem is that some functions cast u32 return values to usize. In most cases that should be fine because the return value is limited by the length of the input buffer and thus fits into a usize. One notable exemption is seek. But let’s review and fix that separately.