ci: Add job for building gccrs with older gcc

As Thomas Schwinge pointed out, GCC 4.8 is the minimum version to be
used for building current GCC, meaning that we should make an effort to
support it before we consider upstreaming or backporting. The main
differences are probably a less powerful standard template library or
small compilation differences.

.github/workflows/ccpp.yml

@@ -84,3 +84,45 @@ jobs:
             else \
               exit 0; \
             fi
+
+  build-and-check-older-gcc:
+
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Install Deps
+      run: |
+          sudo apt-get update;
+          sudo apt-get install -y \
+                  automake \
+                  autoconf \
+                  libtool \
+                  autogen \
+                  bison \
+                  flex \
+                  libgmp3-dev \
+                  libmpfr-dev \
+                  libmpc-dev \
+                  build-essential \
+                  gcc-4.8 \
+                  g++-4.8 \
+                  gcc-4.8-multilib \
+                  g++-4.8-multilib \
+                  dejagnu
+
+    - name: Configure
+      run: |
+           mkdir -p gccrs-build;
+           cd gccrs-build;
+           ../configure \
+               CC='gcc-4.8' \
+               CXX='g++-4.8' \
+               --enable-languages=rust \
+               --disable-bootstrap \
+               --enable-multilib
+
+    - name: Build
+      shell: bash
+      run: |
+           make -C gccrs-build -j $(nproc)