replace inline assembly WebRtcNsx_PrepareSpectrumNeon by intrinsics.

The modification only uses the unique part of the spectrum (as is done for the C and asm code). It passes
byte to byte conformance test, and the single function performance
(if not specified, the code is compiled by GCC 4.6) on different
platforms:

| run 100k times             | cortex-a7 | cortex-a9 | cortex-a15 |
| use C as the base on each  |  (1.2Ghz) |  (1.0Ghz) |   (1.7Ghz) |
| CPU target                 |           |           |            |
|----------------------------+-----------+-----------+------------|
| C                          |      100% |      100% |       100% |
| Neon asm                   |       18% |       14% |        19% |
| Neon inline asm            |       31% |       25% |        27% |
| Neon intrinsic (GCC 4.6)   |       33% |       27% |        42% |
| Neon intrinscis (GCC 4.8)  |       17% |       14% |        19% |
| Neon intrinsics (LLVM 3.3) |       15% |       13% |        18% |

BUG=
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13739004

Patch from Joe Yu <joe.yu@arm.com>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6920 4adac7df-926f-26a2-2b94-8c16560cd09d