Fixes to Handling Multiple Architectures (#2261)

* Added some notes about how multiarch detection could be fixed.

* Implemented a preliminary version of the "spack.spec.ArchSpec" class.

* Updated the "spack.spec.Spec" class to use "ArchSpec" instead of "Arch".

* Fixed a number of small bugs in the "spack.spec.ArchSpec" class.

* Fixed the 'Concretizer.concretize_architecture' method so that it uses the new architecture specs.

* Updated the package class to properly use arch specs.
Removed a number of unused architecture functions.

* Fixed up a number of bugs that were causing the regression tests to fail.
Added a couple of additional regression tests related to architecture parsing/specification.
Fixed a few bugs with setting reserved os/target values on "ArchSpec" objects.
Removed a number of unnecessary functions in the "spack.architecture" and "spack.concretize" modules.

* Fixed a few bugs with reading architecture information from specs.
Updated the tests to use a uniform architecture to improve reliability.
Fixed a few minor style issues.

* Adapted the compiler component of Spack to use arch specs.

* Implemented more test cases for the extended architecture spec features.
Improved error detection for multiple arch components in a spec.

* Fix for backwards compatibility with v0.8 and prior

* Changed os to unknown for compatibility specs

* Use `spack09` instead of `spackcompat` for the platform of old specs.