Add CI workflow for Go legacy versions

Author: lemire

.github/workflows/legacy86.go

@@ -0,0 +1,18 @@
+name: Go-legacy-CI
+on: [push, pull_request]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go-version: ['1.20.x', '1.21.x']
+    steps:
+      - name: Install Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.go-version }}
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Test on 386
+        run: |
+          GOARCH=386 go test