Correctly set -stdlib=libc++ when building with a custom libc++.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index f38b5a3bfd50ef2fc47581e3dd9a826ecd3ddc82..c16b20ff0f64c8e16387f44dc041ffcb649704f1 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -429,16 +429,15 @@ template("gcc_toolchain") {
         use_gold = invoker.use_gold
       }
 
-      if (defined(invoker.clear_sanitizers) && invoker.clear_sanitizers) {
-        is_asan = false
-        is_cfi = false
-        is_lsan = false
-        is_msan = false
-        is_syzyasan = false
-        is_tsan = false
-        is_ubsan = false
-        is_ubsan_vptr = false
-      }
+      # Disable sanitizers for non-default toolchains.
+      is_asan = false
+      is_cfi = false
+      is_lsan = false
+      is_msan = false
+      is_syzyasan = false
+      is_tsan = false
+      is_ubsan = false
+      is_ubsan_vptr = false
     }
 
     forward_variables_from(invoker, [ "deps" ])