Fixed missing / bad header when cross-compiling to i686

It seems that by default the 32-bit headers are not actually installed
when installing development tooling. As we're using gcc headers,
we need to install them as an extra package.

See for reference:
- https://stackoverflow.com/a/54082790
- https://askubuntu.com/a/106092

Also removed the now unused arm tooling

src/ci/docker/host-x86_64/x86_64-gnu-llvm-9/Dockerfile

@@ -2,7 +2,7 @@ FROM ubuntu:18.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
   g++ \
-  g++-arm-linux-gnueabi \
+  gcc-multilib \
   make \
   ninja-build \
   file \