newlib: correct signature of _open() for xtensa

Previously, in libc-hooks.c, the signature of `_open()` was as
shown below.

```cpp
int _open(const char *name, int mode);
```

This conflicted with the signature of `_open()` from newlib,
which is

```cpp
int _open(const char *name, int flags, ...);
```

Moreover, the mode and flags field were reversed, but only for
the Xtensa architecture due to the `_open_r()` hook that is
present in `libc-hooks.c`.

This manifested itself via a call to `fopen(file, "w+")`, where
the expected flags should include `O_CREAT | O_TRUNC`, or
`0x200 | 0x400`. Instead, the unexpected flags passed to the
underlying `zvfs_open()` call were `0x1b6`.

This change corrects the function signature and order of the
arguments.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>