Reflow src/CMakeLists.txt in logical groups

[alexreinking]

1. Move core features closer to Halide library definition.
2. Move target export script to its own section.
3. Group LLVM backends and GPU backends together.

The next four PRs are all real/functional, I promise.

[alexreinking]

Cancelling -make workflows because only src/CMakeLists.txt was touched.

[alexreinking]

mullapudi2016_reorder flaky...