Add C++ to cross-compiling Docker images

This is a follow-on from #512 / d0ed97f where we started always setting
the CXX environment variable to ensure we don't accidentally try and use
the host (non-cross-compiling) C++ compiler.

In attempting to upgrade to a more modern s509x minimum arch-level
(z10), we found that the configure script for bdb fails. It tries to
check the C++ compiler is working (respecting the CXX environment
variable that is set), but fails to run the pre-processor with
`$CXX -E` (as the C++ compiler isn't actually installed). It then falls
back to `/lib/cpp`, which up until now has been working fine. Now that
we're specifying `-march=z10` this fails as `/lib/cpp` doesn't know
about `z10`.

I don't know if we actually need to use the C++ compiler, but I'm adding
it in to at least satisfy the configure script, and do the correct thing
should we need it (rather than the incorrect thing of falling back to
the host compiler).

Author: Edward-Knight

cpython-unix/build.cross-riscv64.Dockerfile

@@ -48,6 +48,15 @@ RUN apt-get install \
 
 # Cross-building.
 RUN apt-get install \
+    g++-aarch64-linux-gnu \
+    g++-arm-linux-gnueabi \
+    g++-arm-linux-gnueabihf \
+    g++-mips-linux-gnu \
+    g++-mips64el-linux-gnuabi64 \
+    g++-mipsel-linux-gnu \
+    g++-powerpc64le-linux-gnu \
+    g++-riscv64-linux-gnu \
+    g++-s390x-linux-gnu \
     gcc-aarch64-linux-gnu \
     gcc-arm-linux-gnueabi \
     gcc-arm-linux-gnueabihf \

cpython-unix/build.cross.Dockerfile

@@ -51,6 +51,14 @@ RUN apt-get install \
 
 # Cross-building.
 RUN apt-get install \
+    g++-aarch64-linux-gnu \
+    g++-arm-linux-gnueabi \
+    g++-arm-linux-gnueabihf \
+    g++-mips-linux-gnu \
+    g++-mips64el-linux-gnuabi64 \
+    g++-mipsel-linux-gnu \
+    g++-powerpc64le-linux-gnu \
+    g++-s390x-linux-gnu \
     gcc-aarch64-linux-gnu \
     gcc-arm-linux-gnueabi \
     gcc-arm-linux-gnueabihf \