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.

goenv/tools-builtin.go

@@ -0,0 +1,7 @@
+//go:build byollvm
+
+package goenv
+
+func init() {
+	hasBuiltinTools = true
+}