Fix test on 32 bit.

This patch adds "-target x86_64" to the command line. Without this option,
a 32 bit object file would be created on 32 bit machine, resulting in test
failure.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193904 91177308-0d34-0410-b5e6-96231b3b80d8