Auto merge of #1082 - IsaacWoods:core_cvoid, r=alexcrichton

Re-export core::ffi::c_void if it exists

This is the second part of the implementation of [RFC 2521](https://github.com/rust-lang/rfcs/pull/2521), replacing the definition of `c_void` in libc with a re-export of the type from `core::ffi::c_void` on builds it exists for.

This uses the re-export for rustc version `1.31.0` or greater, as `1.30.x` was the current nightly when [the PR for the changes to libcore and libstd](https://github.com/rust-lang/rust/pull/53910) was merged, so I'm assuming the first nightly they will appear in will be `1.31.0`; is this acceptable?

cc rust-lang/rust#53856