Convert cfg syntax to new system

This removes the ability to use `foo(bar)` style cfgs. Switch them to
`foo_bar` or `foo="bar"` instead.

[breaking-change]
auto merge of #17527 : sfackler/rust/cfg-syntax, r=alexcrichton

We'll need a snapshot before we can convert the codebase over and turn on the deprecation warnings.

cc #17490

This is sitting on top of #17506