fix for cpuFromIsa

Otherwise we get errors:

    $ ./run_tests.py --target=generic-4
    Testing ispc: /nfs/home5/evghenii/soft/ispc/ispc
    Warning: No generics #include specified; using examples/intrinsics/sse4.h
    
    
    Using test compiler: Intel(r) SPMD Program Compiler (ispc), 1.7.1dev (build commit 3459c75fbcd47c16 @ 20140708, LLVM 3.4)
    Using C/C++ compiler: clang version 3.4 (tags/RELEASE_34/final 199016) (llvm/tags/RELEASE_34/final 199015)

    Running 16 jobs in parallel. Running 1314 tests.

    Traceback (most recent call last):
          File "./run_tests.py", line 816, in <module>
        L = run_tests(options, args, 1)
      File "./run_tests.py", line 714, in run_tests
        raise OSError(2, 'Some test subprocess has thrown an exception', '')
    OSError: [Errno 2] Some test subprocess has thrown an exception: ''

and 

    $ ./ispc --target=generic-4 tests/test-1.ispc --emit-c++    -o out.cpp
    '??I' is not a recognized processor for this target (ignoring processor)
    '??I' is not a recognized processor for this target (ignoring processor)
    '??I' is not a recognized processor for this target (ignoring processor)
    '??I' is not a recognized processor for this target (ignoring processor)
    '??I' is not a recognized processor for this target (ignoring processor)
    '??I' is not a recognized processor for this target (ignoring processor)