Use -march=native for non-bottle source builds [Linux]

[sjackman]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Use -march=native for source builds on Linux. Brew currently uses -march=core2 by default for both bottle and source builds. Previous behaviour was -march=core2 for bottle builds and -march=native for source builds.

[MikeMcQuaid]

Thanks again @sjackman!

[sjackman]

Thanks for merging, Mike!