libuv: Support compiling with GCC on macOS

These Darwin-specific files include Objective-C code from a macOS framework via a header. Needs proper declaration as Objective-C source for all compilers. AppleClang just lets this slip gracefully, that's why this builds in the first place.

Cherry-pick of https://github.com/libuv/libuv/pull/2808

What do you think, can we potentially add macOS+GCC to CMake's CI?

Fixes: #20620 (closed)
Topic-rename: libuv-macos-gcc