fix: return ENOTSUP instead of -1 for unsupported syscalls

[kateinoigakukun]

Negative errno value is not expected at least by wasi-libc and strerror(-1) returns "Success" as an undefined behavior. To avoid such confusion, it might be better to return ENOTSUP for unimplemented syscalls instead of invalid errno -1.
This change makes browser_wasi_shim work with ruby.wasm.

$ cat main.c
#include <stdio.h>
#include <string.h>

int main(void) {
    printf("%s\n", strerror(-1));
    return 0;
}
$ $WASI_SDK_PATH/bin/clang main.c
$ wasmtime a.out
Success

[bjorn3]

Thanks!