fuchsia: Let linker know where to find libclang_rt.builtin.a

build/config/fuchsia/BUILD.gn

Index: build/config/fuchsia/BUILD.gn
diff --git a/build/config/fuchsia/BUILD.gn b/build/config/fuchsia/BUILD.gn
index 9ec7b11ddc5a2eff38f48ce87aa5c2d6fa7d3411..1f8ee1a0e1cbd18be1f1379d091dfe58878c49a7 100644
--- a/build/config/fuchsia/BUILD.gn
+++ b/build/config/fuchsia/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/fuchsia/config.gni")
 import("//build/config/sysroot.gni")
 
 assert(is_fuchsia)
@@ -22,6 +23,15 @@ config("compiler") {
   }
   asmflags = cflags
 
+  # TODO(thakis): Once Fuchsia's libclang_rt.builtin no longer has upstream
+  # patches, we might want to make tools/clang/scripts/update.py build it
+  # and bundle it with the clang package instead of using the library from
+  # the SDK, https://crbug.com/724204
+  ldflags += [
+    "-resource-dir",
+    rebase_path(fuchsia_sdk, root_build_dir) + "/toolchain_libs/clang/5.0.0",
+  ]
+
   libs = [
     "mxio",
     "magenta",