[libc++] Install missing packages to cross-compile to 32 bits during CI

Author: ldionne

libcxx/utils/ci/Dockerfile

@@ -46,6 +46,7 @@ RUN apt-get update && apt-get install -y bash curl
 
 # Install various tools used by the build or the test suite
 RUN apt-get update && apt-get install -y ninja-build python3 python3-sphinx python3-distutils git gdb
+RUN apt-get update && apt-get install -y libc6-dev-i386 # Required to cross-compile to 32 bits
 
 # Install the most recently released LLVM
 RUN apt-get update && apt-get install -y lsb-release wget software-properties-common