std_detect has cfg(feature = "use_std") but no such feature exists. #909

In std_detect::detect, the non-x86{,_64} code for linux / freebsd is behind feature = "use_std":

I believe this is intended to be feature = "std_detect_file_io"? Or maybe not, because the linux module further cordons off the cpuinfo reading under that, implying it expects that it may exist without this std_detect_file_io:

AFAICT this code is just totally dead, and it's a bit of a large amount of dead code to have sitting in the tree... and I can't find an old commit that even had a use_std feature, so I'm probably just wrong and am unaware of some mode that this code is compiled in — I know about "as a module of libstd", but is there anything else I should be aware of?

If not, even if the code is not production ready, it seems like it should stay compiling.

Anyway, I was gonna submit a PR with this to see if the CI passed at least, but I became worried that if it landed, it would regress #850, which added one of these lines, so I figured I'd ask first.