Optimize lseek in the `tell` case.

Optimize lseek in the `tell` case.

`lseek(x, 0, SEEK_CUR)` has no effect other than to return the current
file offset. The patch here uses a macro with `__builtin_constant_p` to
recognize this case and rewrite it to a library call that uses `fd_tell`
rather than `fd_seek`, so that programs that don't need actual seeking
don't end up importing `fd_seek`.

This is also the first usage of `__wasi_fd_tell` in WASI libc, so this
adds it to undefined-symbols.txt.