debian/rules: Ensure we use SSE instructions in i386

This was causing a test failure

See https://bugzilla.mozilla.org/show_bug.cgi?id=1621900#c3

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Simon, we were discussing with @laney whether it's acceptable to raise the CPU requirements to CPUs with sse2 only.

I personally believe it's a fine trade-off in 2020, considering mozjs is required only by few gjs based apps that wouldn't probably even work in such CPUs, but let us know if you're fine or you've other ideas.