Optimize RGB16 blitH functions with NEON for ARM platform.

Optimize RGB16 blitH functions with NEON for ARM platform.

Here are some performance resultsi on Nexus 9:
SkRGB16BlitterBlitH_neon:
    +--------+-----------+
    |height  |  C/NEON   |
    +--------+-----------+
    |1       | 0.888531  |
    +--------+-----------+
    |8       | 1.231800  |
    +--------+-----------+
    |18      | 1.073327  |
    +--------+-----------+
    |32      | 1.136991  |
    +--------+-----------+
    |76      | 1.174638  |
    +--------+-----------+
    |85      | 1.188551  |
    +--------+-----------+
    |120     | 1.180261  |
    +--------+-----------+
    |128     | 1.183726  |
    +--------+-----------+
    |512     | 1.220806  |
    +--------+-----------+

BUG=skia:

Review URL: https://codereview.chromium.org/1229673008