perfetto: fix ChromeOS build

Don't bother defining what the default stdlib should be for non
hermetic clang: just let the compiler decide.

Bug: 257861740
Change-Id: Ifc3afbac92ce72f0001f41af3336156fa991ffbb
diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index 192bee8..bd21589 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -249,9 +249,11 @@
     not_needed([ "hermetic_clang_suppressions" ])
   }
 
-  if (is_clang && !is_hermetic_clang && !is_wasm) {
-    cflags_cc += [ "-stdlib=" + non_hermetic_clang_stdlib ]
-    ldflags += [ "-stdlib=" + non_hermetic_clang_stdlib ]
+  if (non_hermetic_clang_stdlib != "") {
+    if (is_clang && !is_hermetic_clang && !is_wasm) {
+      cflags_cc += [ "-stdlib=" + non_hermetic_clang_stdlib ]
+      ldflags += [ "-stdlib=" + non_hermetic_clang_stdlib ]
+    }
   }
 
   if (is_lto) {