NE10/DSP/RFFT: optimise RFFT for armv8

intrinsic, LLVM 3.5, -O2
on a57, juno, android
   size|   time in ms   |   boost   |
       |  NE10 |  pffft | pffft/NE10|
       |R2C|C2R|R2C|C2R*|  R2C|  C2R|
     32|145|185|279| 319|1.92x|1.72x|
     64|175|200|239| 279|1.36x|1.39x|
    128|166|185|237| 262|1.42x|1.41x|
    256|197|208|232| 256|1.17x|1.23x|
    512|208|216|254| 270|1.22x|1.25x|
   1024|241|244|260| 278|1.07x|1.14x|
   2048|258|263|332| 322|1.28x|1.22x|
   4096|303|304|388| 353|1.28x|1.16x|
   8192|339|334|424| 426|1.25x|1.27x|

intrinsic, GCC 4.9, -O2
on a57, juno, android
   size|    time in ms  |   boost   |
       |  NE10 |  pffft | pffft/NE10|
       |R2C|C2R|R2C|C2R*|  R2C|  C2R|
     32|174|181|328| 410|1.88x|2.26x|
     64|214|216|270| 338|1.26x|1.56x|
    128|210|197|259| 310|1.23x|1.57x|
    256|232|223|243| 283|1.04x|1.26x|
    512|250|222|263| 307|1.04x|1.38x|
   1024|274|251|272| 304|1.00x|1.20x|
   2048|288|277|314| 353|1.08x|1.27x|
   4096|333|303|349| 379|1.04x|1.25x|
   8192|370|342|424| 452|1.14x|1.31x|

* Ne10 supports scale of output for backward RFFT,
  while pffft doesn't. To normalize the benchmark,
  a scale operation was added to the end of each
  call to pffft.
* pffft C2R FFT costs 410ms when size==32, 338ms when
  size==64, this is because the former loops more times
  than the latter does, so it does not mean pffft cost
  more time for short input.

intrinsic, GCC 4.9, -O2
on a53, juno, android
   size|    time in ms  |   boost   |
       |  NE10 |  pffft | pffft/NE10|
       |    R2C|     R2C|        R2C|
     32|    347|     607|      1.74x|
     64|    389|     489|      1.25x|
    128|    334|     484|      1.44x|
    256|    401|     456|      1.13x|
    512|    380|     502|      1.32x|
   1024|    460|     512|      1.11x|
   2048|    481|     593|      1.23x|
   4096|    605|     709|      1.17x|
   8192|    704|     891|      1.26x|

Change-Id: Ide0b974620ae8d06cfa862769004b2110abaaeff