tl;dr; Shouldn't we be calling `LLVMShutdown()` somewhere?

I had a program which was calling: 
```
eglInitialize(disp, &majoir, &minor); 
eglTerminate(disp); 
eglInitialize(disp, &majoir, &minor);
```

Which lead to mesa outputting the following before continuing on normally
during the second call to `eglInitalize`:

```
mesa: for the -simplifycfg-sink-common option: may only occur zero or one
times!
mesa: for the -global-isel-abort option: may only occur zero or one times!
```

This prompted an investigation me to investigate, and with the aid of @imirkin,
I think I got the cause.

You see, in an `eglInitialize` call `radeonsi_dri.so` gets loaded, which also
loads in `libLLVM-9svn.so`, and eventually `ac_init_llvm_once()` gets called
more than once.

That's OK, for as the name implies, it only happens once.

Issue is, when `eglTerminate` is called, `radeonsi_dri.so` gets unloaded, but
not `libLLVM-9svn.so`. This means that the next time we all `eglInitialize`,
LLVM's `ManagedStatic`s will be preserved, but not our static var
`ac_init_llvm_target_once_flag`.

We can fix this by calling `LLVMShutdown` before unloading `radeonsi_dri.so`,
that way LLVM's `ManagedStatic`s are also reset.

Alas, I'm not familiar enough with the mesa codebase, so I got no clue where
the appropriate place to plop this function is. Any aid would be well
appreciated.