Add emscripten support to loader
Related:
* https://github.com/Dav1dde/glad-web/issues/12
* https://github.com/Dav1dde/glad/issues/152

Adding automatic emscripten support to the builtin loader using `emscripten_GetProcAddress`:

```c
#if defined(__EMSCRIPTEN__)
    extern void* emscripten_GetProcAddress(const char *name_);
#endif
```