Ignore stderr from the linker when checking from gold

This was manifesting as a cmake warning when building external clients.
When gcc is using ld.bfd, it calls collect2 which always prints to
stderr, so there's no reason to check it.

Fixes issue 1109.

Review URL: https://codereview.appspot.com/7512043

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1973 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6
Merge branch 'master' into c++11-user-literals