auto merge of #20606 : alexcrichton/rust/stabilize-libc, r=brson

auto merge of #20606 : alexcrichton/rust/stabilize-libc, r=brson

This commit prepares the liblibc library to be moved to crates.io. Unlike the
log, serialize, term, etc crates, the source for this crate will *not* be
duplicated out-of-tree. Instead a new rust-lang/libc repository will be created
with a submodule to this repository and it will use the source directly.

In order to compile within the stable ecosystem of Rust, this crate cannot link
to libcore, and it also needs some tweaks for the other attributes that it has.
As a result this commit tweaks the source of the crate to link to libcore when
built in tree but link to libstd when built via cargo.

Note that the rust-lang/libc crate isn't quite prepared just yet, there's a
Cargo bug or two that I'd like to iron out before publishing it. This is simply
preparing the in-tree source.