rustbuild: Add support for compiletest test suites

rustbuild: Add support for compiletest test suites

This commit adds support in rustbuild for running all of the compiletest test
suites as part of `make check`. The `compiletest` program was moved to
`src/tools` (like `rustbook` and others) and is now just compiled like any other
old tool. Each test suite has a pretty standard set of dependencies and just
tweaks various parameters to the final compiletest executable.

Note that full support is lacking in terms of:

* Once a test suite has passed, that's not remembered. When a test suite is
  requested to be run, it's always run.
* The arguments to compiletest probably don't work for every possible
  combination of platforms and testing environments just yet. There will likely
  need to be future updates to tweak various pieces here and there.
* Cross compiled test suites probably don't work just yet, support for that will
  come in a follow-up patch.