Fix no-std check

Use no-std target to ensure we don't link to std.

.github/workflows/ci.yml

@@ -38,6 +38,7 @@ jobs:
       - name: Install Rust
         # --no-self-update is necessary because the windows environment cannot self-update rustup.exe.
         run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
+      - run: rustup target add thumbv7m-none-eabi
       - name: Install wasm-pack
         uses: taiki-e/install-action@wasm-pack
         if: startsWith(matrix.os, 'ubuntu')
@@ -49,7 +50,8 @@ jobs:
         run: cargo check -Z features=dev_dep
       - run: cargo test
       - name: Build with no default features
-        run: cargo build --no-default-features
+        # Use no-std target to ensure we don't link to std.
+        run: cargo build --no-default-features --target thumbv7m-none-eabi
       - name: Test wasm
         run: wasm-pack test --headless --chrome
         if: startsWith(matrix.os, 'ubuntu')