don't force-on c feature when working in rustc workspace

[RalfJung]

libstd turns on c in it's Cargo.toml. That should be enough. Is there a way to verify this?

Fixes #288

[alexcrichton]

FWIW after some digging alloc as well will need to enable the c feature

[RalfJung]

I could mirror std's setup -- but care to elaborate why? Shouldn't it be enough to do that in one place?

[alexcrichton]

There's a branch in rustbuild which special cases no-std targets where we only compile the alloc crate and its dependencies for those target, not std and below. The std crate by default enables the C feature of compiler-builtins but the alloc crate doesn't currently. It's also fine to just centralize the logic in the alloc crate though.