Add SSE2/MMX build flags when building on an x86 host. This is necessary because some files use SSE2 extensions, which may not be available on all x86 CPUs.
Add SSE2/MMX build flags when building on an x86 host. This is necessary
because some files use SSE2 extensions, which may not be available on
all x86 CPUs.

BUG=607938
R=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/1952043003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392206}

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