gn: use bundled binutils via -B

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index d66f54c57a0c84cbe2f44c46f87d35a3d74eb599..3b50cfb49153eb1562c3bd6bb4779c9f22fa2028 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -14,6 +14,12 @@ declare_args() {
   # Normally, Android builds are lightly optimized, even for debug builds, to
   # keep binary size down. Setting this flag to true disables such optimization
   android_full_debug = false
+
+  # Whether to use the binary binutils checked into third_party/binutils.
+  # These are not multi-arch so cannot be used except on x86 and x86-64 (the
+  # only two architectures that are currently checked in). Turn this off when
+  # you are using a custom toolchain and need to control -B in cflags.
+  linux_use_bundled_binutils = is_linux && cpu_arch == "x64"
 }
 
 use_gold = is_linux && cpu_arch == "x64"
@@ -24,11 +30,7 @@ if (!is_win) {
   # with some utilities such as icecc and ccache. Requires gold and
   # gcc >= 4.8 or clang.
   # http://gcc.gnu.org/wiki/DebugFission
-  #
-  # TODO(GYP) enable this. Currently this gives errors from objcopy, presumably
-  # because some other symbol or toolchain setting isn't correct.
-  #use_debug_fission = use_gold
-  use_debug_fission = false
+  use_debug_fission = use_gold && linux_use_bundled_binutils
 }
 
 # default_include_dirs ---------------------------------------------------------
@@ -267,6 +269,12 @@ config("compiler") {
     ]
   }
 
+  if (linux_use_bundled_binutils) {
+    binutils_path = rebase_path("//third_party/binutils/Linux_x64/Release/bin",
+                                root_build_dir)
+    cflags += [ "-B$binutils_path" ]
+  }
+
   # Clang-specific compiler flags setup.
   # ------------------------------------
   if (is_clang) {