Fix test/Driver/cl-x86-flags.c by providing explicit --target

This isn't very neat, but we haven't found any better ways to
make this test work with non-X86 default target.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213769 91177308-0d34-0410-b5e6-96231b3b80d8