build: Added a workflow to cross-build targetting AArch64

The result of the workflow can be seen in Checks tab of a PR.
Two targets have been added:
- stable aarch64-unknown-linux-gnu
- stable aarch64-unknown-linux-musl

Note: a temporary step was added before building. We used "sed" command
to remove "with-serde" feature of kvm-bindings in vmm/Cargo.toml.
This step should be removed in future when kvm-bindings is ready.

Signed-off-by: Michael Zhao <michael.zhao@arm.com>

.github/workflows/cross-build.yaml

@@ -0,0 +1,32 @@
+name: Cloud Hypervisor Cross Build
+on: [pull_request, create]
+
+jobs:
+  build:
+    if: github.event_name == 'pull_request'
+    name: Build
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - stable
+        target:
+          - aarch64-unknown-linux-gnu
+          - aarch64-unknown-linux-musl
+    steps:
+      - name: Code checkout
+        uses: actions/checkout@v2
+      - name: Install Rust toolchain (${{ matrix.rust }})
+        uses: actions-rs/toolchain@v1
+        with:
+            toolchain: ${{ matrix.rust }}
+            target: ${{ matrix.target }}
+            override: true
+      - name: Disable "with-serde" in kvm-bindings
+        run: sed -i 's/"with-serde",\ //g' vmm/Cargo.toml
+      - name: Build
+        uses: actions-rs/cargo@v1
+        with:
+          use-cross: true
+          command: build
+          args: --target=${{ matrix.target }} --no-default-features --features "mmio"