Refactor matrix definitions

The various build matrix definitions used to be duplicated across
multiple jobs. This changes the matrix definitions to have a single
definition for every OS-Arch target, removing duplication. In addition,
this change makes it easier to set up a separate arm64 Windows build.

The end goal is to have a single matrix definition for every platform
and remove many of the inputs to the `swift-toolchain` sub-build. Once
the Mac build is complete, it should be possible to merge both Windows
and Mac matrix definitions, reducing build complexity.