The script that prints new snapshots for Cargo now uses the same download module
as the rest of Cargo. Also adds a quiet option to suppress printing.

I saw the usage of `call` locking up quite a bit when running this script on
Windows, and googling around on the internet seems to indicate that Popen should
be used instead of call to prevent this locking up, and it does indeed work!

The rustlib folder is in `bin` on Windows, not in `lib`.

We're not actually using it as we're not running the installation script, so no
need to modify it.

Cargo will soon support testing the cross compilation capabilities between 32/64
bit Windows, meaning that there's not "one true value" for any of these
environment variables that Cargo is setting (e.g. where to find `cl`, `link`,
etc). Instead, all dependencies have been updated to probe the system (in the
same manner as the compiler) for the tools that they're using.

All of the logic is housed in the `gcc` crate which now exposes a function to
probe the system for a particular tool. The updated crates here then use the
result of this probe to run the various build scripts. This all boils down to
being able to build MSVC targets inside an MSYS shell instead of requiring use
of a MSVC shell (which doesn't allow for simultaneous host builds and cross
builds).

This commit enables Cargo's bots to test cross builds between 32 and 64 bit
hosts. Many tests were fixed to work on Windows, and the 32-bit test case now
actually crosses to 64.