Use llvm-ar when building with Clang.

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 148 matching lines @@
   } else if (is_android) {
     configs += [ "//build/config/android:compiler" ]
   } else if (is_linux) {
     configs += [ "//build/config/linux:compiler" ]
   } else if (is_nacl) {
     configs += [ "//build/config/nacl:compiler" ]
   } else if (is_ios || is_mac) {
     configs += [ "//build/config/mac:compiler" ]
   }
 
-  # Applies to all Posix systems.
-  if (is_posix) {
-    configs += [ "//build/config/posix:compiler" ]
-  }
-
   # See the definitions below.
   configs += [
     ":compiler_cpu_abi",
     ":compiler_codegen",
   ]
 
   # In general, Windows is totally different, but all the other builds share
   # some common GCC configuration.
   if (!is_win) {
     # Common GCC compiler flags setup.
@@ skipping 112 matching lines @@
       } else {
         cflags += [ "-funwind-tables" ]
       }
     }
   }
 
   # Linux/Android common flags setup.
   # ---------------------------------
   if (is_linux || is_android) {
     if (use_pic) {
-      cflags += [
-        "-fPIC",
-      ]
-      ldflags += [
-        "-fPIC",
-      ]
+      cflags += [ "-fPIC" ]
+      ldflags += [ "-fPIC" ]
     }
 
-    cflags += [
-      "-pipe",  # Use pipes for communicating between sub-processes. Faster.
-    ]
+    cflags += [ "-pipe" ]  # Use pipes for communicating between sub-processes. Faster.

[Nico, 2017/03/27 17:10:39]

Nit: put comment above += line (yes, not your code)

[krasin1, 2017/03/27 17:51:23]

Done.

 
     ldflags += [
       "-Wl,-z,noexecstack",
       "-Wl,-z,now",
       "-Wl,-z,relro",
     ]
     if (!using_sanitizer) {
       if (!use_cfi_diag) {
         ldflags += [ "-Wl,-z,defs" ]
       }
@@ skipping 1387 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }