cmd/tailscaled, net/tstun: build for aix/ppc64

At least in userspace-networking mode.

Fixes #11361

Change-Id: I78d33f0f7e05fe9e9ee95b97c99b593f8fe498f2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -254,9 +254,6 @@ jobs:
             goarch: amd64
           - goos: openbsd
             goarch: amd64
-          # Plan9 (disabled until 3p dependencies are fixed)
-          # - goos: plan9
-          #   goarch: amd64
 
     runs-on: ubuntu-22.04
     steps:
@@ -305,6 +302,47 @@ jobs:
         GOOS: ios
         GOARCH: arm64
 
+  crossmin: # cross-compile for platforms where we only check cmd/tailscale{,d}
+    strategy:
+      fail-fast: false # don't abort the entire matrix if one element fails
+      matrix:
+        include:
+          # Plan9
+          - goos: plan9
+            goarch: amd64
+          # AIX
+          - goos: aix
+            goarch: ppc64
+
+    runs-on: ubuntu-22.04
+    steps:
+    - name: checkout
+      uses: actions/checkout@v4
+    - name: Restore Cache
+      uses: actions/cache@v3
+      with:
+        # Note: unlike the other setups, this is only grabbing the mod download
+        # cache, rather than the whole mod directory, as the download cache
+        # contains zips that can be unpacked in parallel faster than they can be
+        # fetched and extracted by tar
+        path: |
+          ~/.cache/go-build
+          ~/go/pkg/mod/cache
+          ~\AppData\Local\go-build
+        # The -2- here should be incremented when the scheme of data to be
+        # cached changes (e.g. path above changes).
+        key: ${{ github.job }}-${{ runner.os }}-${{ matrix.goos }}-${{ matrix.goarch }}-go-2-${{ hashFiles('**/go.sum') }}-${{ github.run_id }}
+        restore-keys: |
+          ${{ github.job }}-${{ runner.os }}-${{ matrix.goos }}-${{ matrix.goarch }}-go-2-${{ hashFiles('**/go.sum') }}
+          ${{ github.job }}-${{ runner.os }}-${{ matrix.goos }}-${{ matrix.goarch }}-go-2-
+    - name: build core
+      run: ./tool/go build ./cmd/tailscale ./cmd/tailscaled
+      env:
+        GOOS: ${{ matrix.goos }}
+        GOARCH: ${{ matrix.goarch }}
+        GOARM: ${{ matrix.goarm }}
+        CGO_ENABLED: "0"
+
   android:
     # similar to cross above, but android fails to build a few pieces of the
     # repo. We should fix those pieces, they're small, but as a stepping stone,