Use llvm-ar when building with Clang.

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 916d78a120b4e470fdebafc087f7292898ff9824..6a445bda32fe856b741ef288fe6abdbb680a2bb4 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -168,11 +168,6 @@ config("compiler") {
     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",
@@ -309,7 +304,8 @@ config("compiler") {
       ldflags += [ "-fPIC" ]
     }
 
-    cflags += [ "-pipe" ]  # Use pipes for communicating between sub-processes. Faster.
+    # Use pipes for communicating between sub-processes. Faster.
+    cflags += [ "-pipe" ]
 
     ldflags += [
       "-Wl,-z,noexecstack",