picolibc: add include directory to build artefact

This is really just a preparatory commit for musl support. The idea is
to store not just the archive file (.a) but also an include directory.
This is optional for picolibc but required for musl, so the main purpose
of this commit is the refactor needed for this change.

builder/build.go

@@ -86,6 +86,30 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 	}
 	defer os.RemoveAll(dir)
 
+	// Check for a libc dependency.
+	// As a side effect, this also creates the headers for the given libc, if
+	// the libc needs them.
+	root := goenv.Get("TINYGOROOT")
+	var libcDependencies []*compileJob
+	switch config.Target.Libc {
+	case "picolibc":
+		libcJob, err := Picolibc.load(config, dir)
+		if err != nil {
+			return err
+		}
+		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); os.IsNotExist(err) {
+			return errors.New("could not find wasi-libc, perhaps you need to run `make wasi-libc`?")
+		}
+		libcDependencies = append(libcDependencies, dummyCompileJob(path))
+	case "":
+		// no library specified, so nothing to do
+	default:
+		return fmt.Errorf("unknown libc: %s", config.Target.Libc)
+	}
+
 	optLevel, sizeLevel, _ := config.OptLevels()
 	compilerConfig := &compiler.Config{
 		Triple:          config.Triple(),
@@ -489,7 +513,7 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 	// Add compiler-rt dependency if needed. Usually this is a simple load from
 	// a cache.
 	if config.Target.RTLib == "compiler-rt" {
-		job, err := CompilerRT.load(config.Triple(), config.CPU(), dir)
+		job, err := CompilerRT.load(config, dir)
 		if err != nil {
 			return err
 		}
@@ -499,7 +523,6 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 	// Add jobs to compile extra files. These files are in C or assembly and
 	// contain things like the interrupt vector table and low level operations
 	// such as stack switching.
-	root := goenv.Get("TINYGOROOT")
 	for _, path := range config.ExtraFiles() {
 		abspath := filepath.Join(root, path)
 		job := &compileJob{
@@ -538,26 +561,8 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 		ldflags = append(ldflags, lprogram.LDFlags...)
 	}
 
-	// Add libc dependency if needed.
-	switch config.Target.Libc {
-	case "picolibc":
-		job, err := Picolibc.load(config.Triple(), config.CPU(), dir)
-		if err != nil {
-			return err
-		}
-		linkerDependencies = append(linkerDependencies, job)
-	case "wasi-libc":
-		path := filepath.Join(root, "lib/wasi-libc/sysroot/lib/wasm32-wasi/libc.a")
-		if _, err := os.Stat(path); os.IsNotExist(err) {
-			return errors.New("could not find wasi-libc, perhaps you need to run `make wasi-libc`?")
-		}
-		job := dummyCompileJob(path)
-		linkerDependencies = append(linkerDependencies, job)
-	case "":
-		// no library specified, so nothing to do
-	default:
-		return fmt.Errorf("unknown libc: %s", config.Target.Libc)
-	}
+	// Add libc dependencies, if they exist.
+	linkerDependencies = append(linkerDependencies, libcDependencies...)
 
 	// Strip debug information with -no-debug.
 	if !config.Debug() {