Rework CMake support in buildbot

[steven-johnson]

In anticipation of halide/Halide#4644 landing, we need better CMake coverage on our buildbots. This refactors to:

• Pull all the CMake support for building Halide into a non-windows-specific chunk
• Smarten up the use of CTest to more fully test everything we want to test
• Hopefully streamline the code some
• Use Ninja everywhere possible

I'm sure this will take a few tries to get right (we'll have to take down the buildbots to try it out, so scheduling suggestions welcomed) -- offering this up for initial review and comments.

(Note that I wrote this with an eye toward the code in the branch above; it's possible that it may have glitches with our current CMake code in addition to general errors...)

attn @alexreinking

[alexreinking]

Prefer running cmake --build . --config <config> -j <cpus> to ninja for two reasons:

1. ninja uses all cores by default - I'm not sure if this is desirable.
2. The cmake --build ... command works regardless of which generator is being used.

The --config <config> part is only mandatory when building with a multi-config generator like VS or Xcode.

[abadams]

LGTM so I'll hit approve, but I'm not confident I'd find issues. I'll defer to Alex.

[abadams]

Oh, I guess it's still a draft so I can't hit approve. LGTM regardless.

[alexreinking]

Pls squash before merging