github/workflows: Add workflow to build and run unix port on MIPS.

This adds a coverage build and running of the test suite on a MIPS 32-bit
big endian architecture.  It uses the feature of qemu to execute foreign
code as though it were native to the system (using qemu user mode).  The
code compiled for MIPS will run under the qemu VM, but all syscalls made by
this code go to the host (Linux) system.

See related #7268 and #7273.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/ports_unix.yml

@@ -186,3 +186,17 @@ jobs:
     - name: Print failures
       if: failure()
       run: tests/run-tests.py --print-failures
+
+  qemu_mips:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install packages
+      run: source tools/ci.sh && ci_unix_qemu_mips_setup
+    - name: Build
+      run: source tools/ci.sh && ci_unix_qemu_mips_build
+    - name: Run main test suite
+      run: source tools/ci.sh && ci_unix_qemu_mips_run_tests
+    - name: Print failures
+      if: failure()
+      run: tests/run-tests.py --print-failures