posix: signal: extend strsignal buf to cover entire INT range

extends the char buffer in the strsignal function to cover the
entire range of `int`

Had to use `-INT_MAX` as the compiler resolves

```
STRINGIFY(INT_MIN)
```

to:

```
(-2147483647 - 1)
```

instead of:

```
-2147483648
```

Signed-off-by: Yong Cong Sin <ycsin@meta.com>