libvpx: Manually set output dir of gen asm offsets libs.

Force libvpx_asm_offsets_vp8.a and libvpx_asm_offsets_vpx_scale.a
to be built in "shared_generated_dir".

BUG=377062
TBR=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/302863004

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@273304 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
2 files changed