Bug 763495 - Update libvpx source to 1.2.0. r=cpearce,glandium

This updates our in-tree copy of libvpx to match the 1.2.0 git
tag. All but one of the patches we were carrying are in this
upstream version. Our update.sh script should copy the new
files needed but will not remove the old ones for you.

Runtime cpu detection was rewritten upstream. We now generate
per-platform headers for this and include the correct one from
a vpx_rtcd.h wrapper like we were already doing for vpx_config.h

This revision includes improved assembly optimizations and should
be faster on all platforms.

Includes work by Jan Gerber and Ralph Giles.

--HG--
rename : media/libvpx/vp8/common/arm/neon/save_neon_reg.asm => media/libvpx/vp8/common/arm/neon/save_reg_neon.asm
rename : media/libvpx/vp8/common/arm/armv6/vp8_mse16x16_armv6.asm => media/libvpx/vp8/encoder/arm/armv6/vp8_mse16x16_armv6.asm
rename : media/libvpx/vp8/common/arm/neon/vp8_mse16x16_neon.asm => media/libvpx/vp8/encoder/arm/neon/vp8_mse16x16_neon.asm
rename : media/libvpx/vp8/encoder/x86/x86_csystemdependent.c => media/libvpx/vp8/encoder/x86/vp8_enc_stubs_mmx.c
rename : media/libvpx/vpx_config_arm-linux-gcc.c => media/libvpx/vpx_config_armv7-android-gcc.c
rename : media/libvpx/vpx_config_arm-linux-gcc.h => media/libvpx/vpx_config_armv7-android-gcc.h