zig libc: modify errno helper to eliminate `@intCast`
The vast majority of libc functions return `c_int` for the return value,
when setting errno. This utility function is for those cases.
Other cases can hand-roll the logic, or additional helpers can be added.
a2ea36a517Andrew Kelley committed on 1/24/2026, 7:18:18 PMparent9d63dfa