[X86] Don't use NOPL when the assembler is passed an empty CPU string.

This recommits the change from r321026. I have a fix for the lld test now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321038 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed