Fix compile warnings in libc-wasi posix.c:
```
posix.c:880:41: warning: comparison of integers of different signs:
'unsigned long' and 'ssize_t' (aka 'long') [-Wsign-compare]
if (bufoff + iov[i].buf_len < len) {
posix.c:1359:32: warning: comparison of integers of different signs:
'off_t' (aka 'long long') and 'unsigned long long' [-Wsign-compare]
if (ret == 0 && sb.st_size < offset + len)
```
2366e8c493