Fix nginx cross-compilation

INTRO

@shankerwangmiao has prepared a patch that can override uname runtime checks. This patch is merged and used in 1.28.0-5.

And also, @helmutg prepared a patch that solves another part of the cross-compile problem.

And it's just a small step towards making nginx easy to cross-compile.

GOAL

And the goal is to merge this together and enable cross-compilation of nginx.