Docs give the wrong hxcpp flag for debug symbol generation

I'm trying to compile a Haxe library I'm writing with hxcpp so I can debug it. I would try debugging it as JavaScript, but it relies on the `sys` package so that's not trivial.

The problem is that the binary hxcpp comes up with is missing debug symbols according to gdb. I'm building it with the following `hxml` file:

```
-lib hscript
-lib utest
-main tests.TestMain
-cpp cpp
```

And the following steps:
```
haxe debug.hxml -D no-compilation
cd cpp
haxelib run hxcpp Build.xml -DHXCPP_DEBUGGER -DHXCPP_DEBUG_LINK
```

After investigating in the source myself, it seems the culprit might be that the `-g` flag is never referenced in `gcc-toolchain.xml`:

https://github.com/HaxeFoundation/hxcpp/blob/ff463f43bd5f85c042f0f05e73508b15537579f9/toolchain/gcc-toolchain.xml#L40

I'll investigate locally by building from source, and if I'm right, I should be able to PR this.