build: Stop testing LINKER_FEATURES for ifunc and build-

[emaste]

Summary

These features are available in all supported linkers.

[kib]

Would it make sense to test this somewhere on top level, and refuse to start the buildworld and buildkernel?
I am thinking about adding support for other linkers, that do appear. E.g. mold.

[emaste]

Would it make sense to test this somewhere on top level, and refuse to start the buildworld and buildkernel?

I suppose, although I think the two features here (build-id and ifunc) are "table stakes" for any linker to be a credible alternative to GNU ld (like mold). I imagine such a check would mainly serve as a small hurdle (e.g. Error: unknown linker mold) when trying a new linker.