Optimize RGB16 blitV functions with NEON for ARM platform.

Here are some performance resultsi on Nexus 9:
SkRGB16BlitterBlitV_neon:
+--------+-----------+
|height  |  C/NEON   |
+--------+-----------+
|1       | 0.765230  |
+--------+-----------+
|8       | 1.273330  |
+--------+-----------+
|18      | 1.441462  |
+--------+-----------+
|32      | 1.627798  |
+--------+-----------+
|76      | 1.683131  |
+--------+-----------+
|85      | 1.679456  |
+--------+-----------+
|120     | 1.721311  |
+--------+-----------+
|128     | 1.725482  |
+--------+-----------+
|512     | 1.784117  |
+--------+-----------+

BUG=skia:

Review URL: https://codereview.chromium.org/1213723002
3 files changed