llvm6: Update FreeBSD images to Ubuntu 18.04

Looks like the clang with 16.04 fails to compile LLVM 6, but it looks like clang
in 18.04 can indeed compile LLVM 6.

src/ci/docker/dist-i686-freebsd/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
   clang \

src/ci/docker/dist-x86_64-freebsd/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
   clang \

src/llvm-emscripten

@@ -0,0 +1 @@
+Subproject commit 2717444753318e461e0c3b30dacd03ffbac96903