Fix libvpx's gyp for enabling AVX instructions with MSVC and clang-cl

Fix libvpx's gyp for enabling AVX instructions with MSVC and clang-cl

The correct enum value for AVX is 3, not 4. The enum value of 4 implies
/arch:IA32, which means x87 FPU ops only.

R=johannkoenig@google.com,tomfinegan@chromium.org,hans@chromium.org
BUG=557968
Committed: https://crrev.com/3ee81c7e4cd918089b93b144024b717168f43f2c
Cr-Commit-Position: refs/heads/master@{#360455}

[hans, 2015-11-18 22:36:06]

LGTM

[Johann, 2015-11-18 22:36:33]

LGTM. Apologies, I just looked at
https://msdn.microsoft.com/en-us/library/7t5yh4fd.aspx and made as assumption.

[hans, 2015-11-18 22:37:34]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2015-11-18 22:38:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1460763003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1460763003/1

[commit-bot: I haz the power, 2015-11-19 00:38:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-19 00:40:18]

Patchset 1 (id:??) landed as
https://crrev.com/3ee81c7e4cd918089b93b144024b717168f43f2c
Cr-Commit-Position: refs/heads/master@{#360455}