Add a crate for missing stubs from libcore

The core library in theory has 0 dependencies, but in practice it has some in
order for it to be efficient. These dependencies are in the form of the basic
memory operations provided by libc traditionally, such as memset, memcmp, etc.
These functions are trivial to implement and themselves have 0 dependencies.

This commit adds a new crate, librlibc, which will serve the purpose of
providing these dependencies. The crate is never linked to by default, but is
available to be linked to by downstream consumers. Normally these functions are
provided by the system libc, but in other freestanding contexts a libc may not
be available. In these cases, librlibc will suffice for enabling execution with
libcore.

cc #10116

src/libcore/lib.rs

@@ -18,6 +18,9 @@
 //! * `memcpy`, `memcmp`, `memset` - These are core memory routines which are
 //!   often generated by LLVM. Additionally, this library can make explicit
 //!   calls to these funcitons. Their signatures are the same as found in C.
+//!   These functions are often provided by the system libc, but can also be
+//!   provided by `librlibc` which is distributed with the standard rust
+//!   distribution.
 //!
 //! * `rust_begin_unwind` - This function takes three arguments, a
 //!   `&fmt::Arguments`, a `&str`, and a `uint. These three arguments dictate