builder: build wasi-libc inside TinyGo

Instead of relying on a build when TinyGo is being built, do it like all
other libraries when it is needed.

This brings a few benefits:

  * No more running `make wasi-libc` on the command line as a special
    case for WebAssembly. The generic `git submodule update --init` step
    is now enough for wasi-libc.
  * It becomes much easier to customize the build per system. For
    example: include/exclude malloc as needed, disable/enable bulk
    memory operations per target, etc.

.github/workflows/windows.yml

@@ -91,15 +91,6 @@ jobs:
         with:
           key: ${{ steps.cache-llvm-build.outputs.cache-primary-key }}
           path: llvm-build
-      - name: Cache wasi-libc sysroot
-        uses: actions/cache@v4
-        id: cache-wasi-libc
-        with:
-          key: wasi-libc-sysroot-v5
-          path: lib/wasi-libc/sysroot
-      - name: Build wasi-libc
-        if: steps.cache-wasi-libc.outputs.cache-hit != 'true'
-        run: make wasi-libc
       - name: Cache Go cache
         uses: actions/cache@v4
         with: