We have previously always relied upon an external tool, `ar`, to modify archives
that the compiler produces (staticlibs, rlibs, etc). This approach, however, has
a number of downsides:

* Spawning a process is relatively expensive for small compilations
* Encoding arguments across process boundaries often incurs unnecessary overhead
  or lossiness. For example `ar` has a tough time dealing with files that have
  the same name in archives, and the compiler copies many files around to ensure
  they can be passed to `ar` in a reasonable fashion.
* Most `ar` programs found do **not** have the ability to target arbitrary
  platforms, so this is an extra tool which needs to be found/specified when
  cross compiling.

The LLVM project has had a tool called `llvm-ar` for quite some time now, but it
wasn't available in the standard LLVM libraries (it was just a standalone
program). Recently, however, in LLVM 3.7, this functionality has been moved to a
library and is now accessible by consumers of LLVM via the `writeArchive`
function.

This commit migrates our archive bindings to no longer invoke `ar` by default
but instead make a library call to LLVM to do various operations. This solves
all of the downsides listed above:

* Archive management is now much faster, for example creating a "hello world"
  staticlib is now 6x faster (50ms => 8ms). Linking dynamic libraries also
  recently started requiring modification of rlibs, and linking a hello world
  dynamic library is now 2x faster.
* The compiler is now one step closer to "hassle free" cross compilation because
  no external tool is needed for managing archives, LLVM does the right thing!

This commit does not remove support for calling a system `ar` utility currently.
We will continue to maintain compatibility with LLVM 3.5 and 3.6 looking forward
(so the system LLVM can be used wherever possible), and in these cases we must
shell out to a system utility. All nightly builds of Rust, however, will stop
needing a system `ar`.

…lexcrichton

Document that `-C ar=PATH` doesn't do anything

Are there any plans to use an external archiver in the future?
IIRC, it was used before, but its use was replaced with LLVM's built-in archive management machinery. I can't found a relevant PR though. EDIT: Found it - rust-lang#26926!

The `-C` option is stable so it still can't be removed right away even if there are no plans to use it (but maybe it can be deprecated?).
Target specifications have a field for archiver as well, which is unused too (these ones are unstable, so I guess it can be removed).

r? @alexcrichton