Architecture aliases: Win32 & Win64 (#29)

By a public request, let's support aliases for architecture parameters.
Treat "arch: Win32" as "x86" and "Win64" as "x64".

Test this on CI just in case x86 breaks or something.

.github/workflows/main.yml

@@ -33,3 +33,20 @@ jobs:
         uses: actions/checkout@v2
       - run: npm install
       - run: npm audit --audit-level=moderate
+  alias-arch:
+    name: arch aliases
+    runs-on: windows-latest
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v2
+      - name: Download Internet
+        run: npm install
+      - name: Enable Developer Command Prompt
+        uses: ./
+        with:
+          arch: Win32
+      - name: Compile and run some C code
+        shell: cmd
+        run: |
+          cl.exe hello.c
+          hello.exe