GitHub Actions: Add cross compile tests

Add cross compile tests without running checks. Currently
only mips64el, riscv64, mipsel, alpha is wired up.
Just help us make sure those less popular CPUs are not
messed up by changes.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

.github/workflows/dynamic_arch.yml

@@ -257,3 +257,53 @@ jobs:
       - name: Run tests
         timeout-minutes: 60
         run: cd build && ctest
+
+  cross_build:
+    runs-on: ubuntu-22.04
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - target: mips64el
+            triple: mips64el-linux-gnuabi64
+            opts: DYNAMIC_ARCH=1
+          - target: riscv64
+            triple: riscv64-linux-gnu
+            opts: TARGET=RISCV64_GENERIC
+          - target: mipsel
+            triple: mipsel-linux-gnu
+            opts: TARGET=MIPS1004K
+          - target: alpha
+            triple: alpha-linux-gnu
+            opts: TARGET=EV4
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - name: Install Dependencies
+        run: |
+          sudo apt-get install -y ccache gcc-${{ matrix.triple }} gfortran-${{ matrix.triple }} libgomp1-${{ matrix.target }}-cross
+
+      - name: Compilation cache
+        uses: actions/cache@v3
+        with:
+          path: ~/.ccache
+          key: ccache-${{ runner.os }}-${{ matrix.target }}-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            ccache-${{ runner.os }}-${{ matrix.target }}-${{ github.ref }}
+            ccache-${{ runner.os }}-${{ matrix.target }}
+
+      - name: Configure ccache
+        run: |
+          # Limit the maximum size and switch on compression to avoid exceeding the total disk or cache quota (5 GB).
+          test -d ~/.ccache || mkdir -p ~/.ccache
+          echo "max_size = 300M" > ~/.ccache/ccache.conf
+          echo "compression = true" >> ~/.ccache/ccache.conf
+          ccache -s
+
+
+      - name: Build OpenBLAS
+        run: |
+          make -j$(nproc) HOSTCC="ccache gcc" CC="ccache ${{ matrix.triple }}-gcc" FC="ccache ${{ matrix.triple }}-gfortran" ARCH=${{ matrix.target }} ${{ matrix.opts }}