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.

builder/build.go

@@ -14,7 +14,6 @@ import (
 	"fmt"
 	"go/types"
 	"hash/crc32"
-	"io/fs"
 	"math/bits"
 	"os"
 	"os/exec"
@@ -168,11 +167,12 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		defer unlock()
 		libcDependencies = append(libcDependencies, libcJob)
 	case "wasi-libc":
-		path := filepath.Join(root, "lib/wasi-libc/sysroot/lib/wasm32-wasi/libc.a")
-		if _, err := os.Stat(path); errors.Is(err, fs.ErrNotExist) {
-			return BuildResult{}, errors.New("could not find wasi-libc, perhaps you need to run `make wasi-libc`?")
+		libcJob, unlock, err := libWasiLibc.load(config, tmpdir)
+		if err != nil {
+			return BuildResult{}, err
 		}
-		libcDependencies = append(libcDependencies, dummyCompileJob(path))
+		defer unlock()
+		libcDependencies = append(libcDependencies, libcJob)
 	case "wasmbuiltins":
 		libcJob, unlock, err := libWasmBuiltins.load(config, tmpdir)
 		if err != nil {