replace inline assembly WebRtcNsx_SynthesisUpdateNeon by intrinsics.

The modification only uses the unique part of the synthesis_update
function. Pass byte to byte conformance test both on ARMv7 and ARMv8,
and the single function performance is similar with original assembly
version on different platforms (if not specified, the code is compiled
by GCC 4.6):

| run 100k times             | cortex-a7 | cortex-a9 | cortex-a15 |
| use C as the base          |  (1.2Ghz) |  (1.0Ghz) |   (1.7Ghz) |
| (the smaller the better)   |           |           |            |
|----------------------------+-----------+-----------+------------|
| C                          |      100% |      100% |       100% |
| Neon asm                   |    15.93% |    17.01% |     12.50% |
| Neon inline asm            |    27.74% |    31.41% |     14.64% |
| Neon intrinsics (GCC 4.8)  |    17.84% |    14.10% |     13.84% |
| Neon intrinsics (LLVM 3.4) |    16.63% |    14.01% |     12.98% |

BUG=3580
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23159004

Patch from Zhongwei Yao <zhongwei.yao@arm.com>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7586 4adac7df-926f-26a2-2b94-8c16560cd09d