Revert part of r321026 "[X86] Don't use NOPL when the assembler is passed an...

Revert part of r321026 "[X86] Don't use NOPL when the assembler is passed an empty CPU string." while I investigate how to fix an lld test failure.

Looks like lld also needs to pass a -mcpu in some of its tests

llvm-svn: 321033