libvpx

build: convert rtcd.sh to perl

significantly speeds up file generation.

the goal of this change is to convert rtcd.sh to perl as directly as
possible to allow for simple comparison. future changes can make it more
perl-like.

---
Linux
    [CREATE] vpx_scale_rtcd.h
real    0m0.485s ->    0m0.022s
    [CREATE] vp8_rtcd.h
real    0m4.619s ->    0m0.060s
    [CREATE] vp9_rtcd.h
real    0m10.102s ->    0m0.087s

Windows
    [CREATE] vpx_scale_rtcd.h
real    0m8.360s ->    0m0.080s
    [CREATE] vp8_rtcd.h
real    1m8.083s ->    0m0.160s
    [CREATE] vp9_rtcd.h
real    2m6.489s ->    0m0.233s

Change-Id: Idfb71188206c91237d6a3c3a81dfe00d103f11ee