Fix nightly CI by adding --check-cfg flags

[jsgf]

Emit cargo::rustc-check-cfg=CHECK_CFG in build.rs so that they can be cross-checked even if cfg option itself isn't set.

[jsgf]

This is a subset of #205, aside from (I think) handling the riscv{ext} cfg flags better.

[jsgf]

Ok rebased and just reduced to using a loop for per-ext checks.