Add ARM32 build test to CI.

We do not yet want to gate our CI on tests passing, because the backend
is only partially complete; but we want to make sure that it remains
up-to-date as we change internal APIs.

.github/workflows/main.yml

@@ -108,9 +108,11 @@ jobs:
     # Check a few builds of the cranelift backend
     # - only x86 backend support,
     # - only arm64 backend support,
+    # - experimental arm32 support,
     # - no debug_assertions.
     - run: cargo check --manifest-path=./cranelift/Cargo.toml --bin clif-util --no-default-features --features=cranelift-codegen/arm64
     - run: cargo check --manifest-path=./cranelift/Cargo.toml --bin clif-util --no-default-features --features=cranelift-codegen/x86
+    - run: cargo check --manifest-path=./cranelift/Cargo.toml --bin clif-util --no-default-features --features=cranelift-codegen/arm32
     - run: cargo check --manifest-path=./cranelift/Cargo.toml --bin clif-util
       env:
         CARGO_PROFILE_DEV_DEBUG_ASSERTIONS: false