Reenable the new backend x64 CI;

The intermittent failure have likely been fixed by a recent round of
general fixes in the new x64 backend. This basically reverts
https://github.com/bytecodealliance/wasmtime/pull/2100 and uses the CI
script there.

.github/workflows/main.yml

@@ -271,6 +271,32 @@ jobs:
       env:
         RUST_BACKTRACE: 1
 
+  # Perform all tests (debug mode) for `wasmtime` with the experimental x64
+  # backend. This runs on the nightly channel of Rust (because of issues with
+  # unifying Cargo features on stable) on Ubuntu.
+  test_x64:
+    name: Test x64 new backend
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: true
+    - uses: ./.github/actions/install-rust
+      with:
+        toolchain: nightly-2020-08-25
+    - uses: ./.github/actions/define-llvm-env
+
+    # Install wasm32 targets in order to build various tests throughout the
+    # repo.
+    - run: rustup target add wasm32-wasi
+    - run: rustup target add wasm32-unknown-unknown
+
+    # Run the x64 CI script.
+    - run: ./ci/run-experimental-x64-ci.sh
+      env:
+        CARGO_VERSION: "+nightly-2020-08-25"
+        RUST_BACKTRACE: 1
+
   # Verify that cranelift's code generation is deterministic
   meta_determinist_check:
     name: Meta deterministic check