Switch default to new x86_64 backend.

This PR switches the default backend on x86, for both the
`cranelift-codegen` crate and for Wasmtime, to the new
(`MachInst`-style, `VCode`-based) backend that has been under
development and testing for some time now.

The old backend is still available by default in builds with the
`old-x86-backend` feature, or by requesting `BackendVariant::Legacy`
from the appropriate APIs.

As part of that switch, it adds some more runtime-configurable plumbing
to the testing infrastructure so that tests can be run using the
appropriate backend. `clif-util test` is now capable of parsing a
backend selector option from filetests and instantiating the correct
backend.

CI has been updated so that the old x86 backend continues to run its
tests, just as we used to run the new x64 backend separately.

At some point, we will remove the old x86 backend entirely, once we are
satisfied that the new backend has not caused any unforeseen issues and
we do not need to revert.

.github/workflows/main.yml

@@ -286,7 +286,7 @@ jobs:
     # Test debug (DWARF) related functionality on new backend.
     - run: |
         sudo apt-get update && sudo apt-get install -y gdb lldb
-        cargo test --features experimental_x64 test_debug_dwarf -- --ignored --test-threads 1 --test debug::
+        cargo test test_debug_dwarf -- --ignored --test-threads 1 --test debug::
       if: matrix.os == 'ubuntu-latest'
       env:
         RUST_BACKTRACE: 1
@@ -309,10 +309,9 @@ jobs:
       env:
         RUST_BACKTRACE: 1
 
-  # Perform all tests (debug mode) for `wasmtime` with the experimental x64
-  # backend.
+  # Perform all tests (debug mode) for `wasmtime` with the old x86 backend.
   test_x64:
-    name: Test x64 new backend
+    name: Test old x86 backend
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
@@ -328,38 +327,8 @@ jobs:
     - 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:
-        RUST_BACKTRACE: 1
-
-  # Perform tests on the new x64 backend on Windows as well.
-  test_x64_win:
-    name: Test x64 new backend on Windows
-    runs-on: windows-latest
-    steps:
-    - uses: actions/checkout@v2
-      with:
-        submodules: true
-    - uses: ./.github/actions/install-rust
-    - uses: ./.github/actions/define-llvm-env
-
-    - name: Install libclang
-      # Note: libclang is pre-installed on the macOS and linux images.
-      if: matrix.os == 'windows-latest'
-      run: |
-        curl https://releases.llvm.org/9.0.0/LLVM-9.0.0-win64.exe -o llvm-installer.exe
-        7z x llvm-installer.exe -oC:/llvm-binary
-        echo LIBCLANG_PATH=C:/llvm-binary/bin/libclang.dll >> $GITHUB_ENV
-        echo C:/llvm-binary/bin >> $GITHUB_PATH
-
-    # 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
+    # Run the old x86 backend CI (we will eventually remove this).
+    - run: ./ci/run-old-x86-ci.sh
       env:
         RUST_BACKTRACE: 1