Support more configurability in GN toolchains

tools/gn/builder.cc

Index: tools/gn/builder.cc
diff --git a/tools/gn/builder.cc b/tools/gn/builder.cc
index d498b108e1bf02d9cc1198c1c676d66b68c2f463..dccc095a949c6658a8211110fa61c1ed2100f108 100644
--- a/tools/gn/builder.cc
+++ b/tools/gn/builder.cc
@@ -408,7 +408,8 @@ bool Builder::ResolveItem(BuilderRecord* record, Err* err) {
         !ResolveConfigs(&target->configs(), err) ||
         !ResolveConfigs(&target->all_dependent_configs(), err) ||
         !ResolveConfigs(&target->direct_dependent_configs(), err) ||
-        !ResolveForwardDependentConfigs(target, err))
+        !ResolveForwardDependentConfigs(target, err) ||
+        !ResolveToolchain(target, err))
       return false;
   } else if (record->type() == BuilderRecord::ITEM_TOOLCHAIN) {
     Toolchain* toolchain = record->item()->AsToolchain();
@@ -499,6 +500,24 @@ bool Builder::ResolveForwardDependentConfigs(Target* target, Err* err) {
   return true;
 }
 
+bool Builder::ResolveToolchain(Target* target, Err* err) {
+  BuilderRecord* record = GetResolvedRecordOfType(
+      target->settings()->toolchain_label(), target->defined_from(),
+      BuilderRecord::ITEM_TOOLCHAIN, err);
+  if (!record) {
+    *err = Err(target->defined_from(),
+        "Toolchain for target not defined.",
+        "I was hoping to find a toolchain " +
+        target->settings()->toolchain_label().GetUserVisibleName(false));
+    return false;
+  }
+
+  if (!target->SetToolchain(record->item()->AsToolchain(), err))
+    return false;
+
+  return true;
+}
+
 std::string Builder::CheckForCircularDependencies(
     const std::vector<const BuilderRecord*>& bad_records) const {
   std::vector<const BuilderRecord*> cycle;