builder: refactor clang include headers

Set -resource-dir in a central place instead of passing the header path
around everywhere and adding it using the `-I` flag. I believe this is
closer to how Clang is intended to be used.

This change was inspired by my attempt to add a Nix flake file to
TinyGo.

builder/tools.go

@@ -1,7 +1,6 @@
 package builder
 
 import (
-	"errors"
 	"os"
 	"os/exec"
 
@@ -12,11 +11,6 @@ import (
 func runCCompiler(flags ...string) error {
 	if hasBuiltinTools {
 		// Compile this with the internal Clang compiler.
-		headerPath := getClangHeaderPath(goenv.Get("TINYGOROOT"))
-		if headerPath == "" {
-			return errors.New("could not locate Clang headers")
-		}
-		flags = append(flags, "-I"+headerPath)
 		cmd := exec.Command(os.Args[0], append([]string{"clang"}, flags...)...)
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr