GN: Build libvpx on POSIX x86

libvpx now builds on POSIX x86. But ARM NEON and Windows still need more
work.

Also fixed the helper script. It now generates .gni source lists and
outputs a correct vpx_config.asm for YASM.

R=brettw@chromium.org, tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/530193002

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@291834 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
23 files changed