Turn on cfg format warnings
Fix cfg warnings for liblibc
Fix liballoc
Fix libstd
Fix librustrt
Fix libtime
Fix librustc_back
Fix librustc
Fix librustc_llvm
Fix libgreen
Fix libnative
Fix librustuv
Fix rustdoc
auto merge of #17630 : sfackler/rust/cfg-warnings, r=brson

Closes #17490