Explicitly link against libatomic for 32bit mips target cpu.

Bug: 821015
Change-Id: I60824857a26ea8660b80aa22dafd33bb3fc42289
Reviewed-on: https://chromium-review.googlesource.com/959120Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542545}

build/config/android/BUILD.gn

@@ -170,6 +170,10 @@ config("runtime_library") {
     libs += [ "atomic" ]
   }
 
+  if (current_cpu == "mipsel") {
+    libs += [ "atomic" ]
+  }
+
   # TODO(jdduke) Re-enable on mips after resolving linking
   # issues with libc++ (crbug.com/456380).
   if (current_cpu != "mipsel" && current_cpu != "mips64el") {