ci: Configure g++ with 32-bit for 32-bit build

In future commits, we'll start to make improvements to C++
infrastructure.  This requires that for our 32-bit builds we also
build C++ code for the 32-bit architecture.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/D98JHQF7H2A8.VSE3I4CJBTAB%40partin.io

.cirrus.tasks.yml

@@ -570,6 +570,7 @@ task:
         su postgres <<-EOF
           set -e
           export CC='ccache gcc -m32'
+          export CXX='ccache g++ -m32'
           meson setup \
             ${MESON_COMMON_PG_CONFIG_ARGS} \
             --buildtype=debug \