optimize int32/int16 complex FFT

    The performance result is as follows:

    toolchain: gcc 4.8 at -O2
    omx fft's execute time is the base. The ratio is less, the performance is better.

    int32 FFT
    A9:
    |     |16    |32    |64    |128   |256   |512   |1024  |2048  |4096  |
    |Ne10 |73.24%|99.95%|95.78%|96.04%|97.97%|97.57%|99.51%|97.87%|98.12%|
    |omx  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |

    A15:
    |     |16    |32    |64    |128   |256   |512   |1024  |2048  |4096  |
    |Ne10 |84.89%|98.62%|89.33%|100.7%|99.28%|103.9%|101.7%|105.1%|96.67%|
    |omx  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |

    int16 FFT
    A9:
    |     |16    |32    |64    |128   |256   |512   |1024  |2048  |4096  |
    |Ne10 |109.2%|97.81%|100.3%|97.20%|101.3%|99.01%|103.4%|103.5%|94.67%|
    |omx  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |

    A15:
    |     |16    |32    |64    |128   |256   |512   |1024  |2048  |4096  |
    |Ne10 |112.6%|95.78%|104.3%|101.7%|112.3%|111.5%|102.3%|105.1%|99.78%|
    |omx  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |100%  |

Change-Id: I7290ae5f9abfd3d04f8ca501f5ecbff452973d4b