swift234

[Toolchains] Cygwin toolchain inherits from Unix

There is currently a great deal of duplication across the
`GenericUnix` and `Windows` toolchains. The Android port will
add even more duplication.

To mitigate this, have `Windows` inherit from `GenericUnix`, and
have them share most of their implementation.

In addition, rename `Windows` to `Cygwin` (it would be pretty strange
to have a `Windows` toolchain inherit from something named `*Unix`).