Don't treat native-compiled functions as built-ins

[flatwhatson]

This fixes the problem where elisp-def hunts for a non-existent C source for native-compiled elisp functions. The subr-primitive-p function is added to subr.el in the native-comp branch of emacs.

[coveralls]

Pull Request Test Coverage Report for Build 54

• 2 of 3 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 90.574%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
elisp-def.el	2	3	66.67%
<!--	Total:	2	3	66.67%	-->

Totals
Change from base Build 53:	-0.2%
Covered Lines:	442
Relevant Lines:	488

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💛 - Coveralls

[Wilfred]

Thanks :)

[flatwhatson]

Thank you!!