Explicitly link against libatomic for 32bit mips linux target.

R=thomasanderson@chromium.org

Bug: 822411
Change-Id: Ib82849f24695049fb62c85f59a7326fb334332d4
Reviewed-on: https://chromium-review.googlesource.com/963592Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543573}

build/config/linux/BUILD.gn

@@ -25,6 +25,10 @@ config("runtime_library") {
   if (is_chromeos) {
     defines = [ "OS_CHROMEOS" ]
   }
+
+  if (current_cpu == "mipsel") {
+    libs = [ "atomic" ]
+  }
 }
 
 config("x11") {