Fix problem with PNaCl glibc bot.
Fix problem with PNaCl glibc bot.

The problem is occuring because the driver is trying to do bitcode linking (gold)
when there's no bitcode inputs.
Review URL: http://codereview.chromium.org/7277052

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@5839 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client