Portability workaround for wasi/api.h errno constants

Uses UINT16_C for backing of errno constants (EPERM etc) as in
previous versions, rather than casting them to __wasm_errno_t.

This fixes some portable codebases that use the C preprocessor
to compare the constant values like this:

```
    #if EPERM > 0
    #define DAV1D_ERR(e) (-(e))
    #else
    #define DAV1D_ERR(e) (e)
    #endif
```

Adds a test case to catch regressions.

Fixes #9996

Author: bvibber