cpu: fix SSE3 detection on Linux

[jonchang]

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For exciting historical reasons, /proc/cpuinfo (sometimes?) lists the SSE3 instructions feature as PNI (Prescott New Instructions). My Haswell test machine (which certainly supports SSE3 instructions) doesn't have sse3 listed but does have pni listed.

c.f. https://github.com/Homebrew/homebrew-science/issues/5920

[reitermarkus]

Use || here.

[MikeMcQuaid]

Thanks for the PR @jonchang!