32 bit thumb2 performance:
NV12ToARGB_Opt (472 ms)
NV21ToARGB_Opt (466 ms)
NV12ToRGB24_Opt (457 ms)
NV21ToRGB24_Opt (457 ms)
NV12ToRGB565_Opt (501 ms)

Bug: libyuv:778
Test: add new NV21ToRGB24 test
Change-Id: I330585789835c79ee4b4da61d164716598268df3
Reviewed-on: https://chromium-review.googlesource.com/924646Reviewed-by: Cheng Wang <wangcheng@google.com>