test: add target to gen coverage report

[inkydragon]

Part of #290

• [x] Add make coverage
• [x] Setup CI, upload HTML cov report
• [x] Upload cov report to coveralls or other online ci service

[codecov[bot]]

Welcome to Codecov :tada:

Once merged to your default branch, Codecov will compare your coverage reports and display the results in this comment.

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[inkydragon]

Sample report: https://app.codecov.io/github/JuliaMath/openlibm/tree/cov-test/

[ViralBShah]

Merge?