Fix V8 gcc build when using libc++

build/config/c++/BUILD.gn

Index: build/config/c++/BUILD.gn
diff --git a/build/config/c++/BUILD.gn b/build/config/c++/BUILD.gn
index 21316ba1c8bd3160c4b08a0a65bea5b72871e6c9..baad310d724d423cada78d237f0f06b2607b2985 100644
--- a/build/config/c++/BUILD.gn
+++ b/build/config/c++/BUILD.gn
@@ -8,6 +8,12 @@ config("c++flags") {
   if (use_custom_libcxx) {
     prefix = "//buildtools/third_party"
     include = "trunk/include"
+    if (!is_clang) {
+      # Gcc has a built-in abs() definition with default visibility.
+      # If it was not disabled, it would conflict with libc++'s abs()
+      # with hidden visibility.
+      cflags = [ "-fno-builtin-abs" ]
+    }
     cflags_cc = [
       "-nostdinc++",
       "-isystem" + rebase_path("$prefix/libc++/$include", root_build_dir),