Default to "ar" instead of "llvm-ar"

My system has a clang but no llvm-ar installed. I can't think of any
reason why the "ar" version should materially affect the output so
default to the more common version.

Change-Id: I4a528f5c22c21378d7105634c7e6d505e34b083e
Reviewed-on: https://gn-review.googlesource.com/2440
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index fce8fb1..9007277 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -247,7 +247,7 @@
     cc = os.environ.get('CC', 'clang')
     cxx = os.environ.get('CXX', 'clang++')
     ld = cxx
-    ar = os.environ.get('AR', 'llvm-ar')
+    ar = os.environ.get('AR', 'ar')
 
   cflags = os.environ.get('CFLAGS', '').split()
   cflags_cc = os.environ.get('CXXFLAGS', '').split()