Add CMakeLists.txt to the following folders under src that create the
following targets:

Device -> vk_device
Pipeline -> vk_pipeline
WSI -> vk_wsi
System -> vk_system
Vulkan -> vk_swiftshader

Dependencies (by folder):

System --> n/a
Pipeline --> System Device* Vulkan*
WSI --> System Pipeline Vulkan*
Device --> System Pipeline Vulkan*
Vulkan --> Device Pipeline WSI System

* = dependency by include directory only (should fix this in the future)

Also make system-unittests and system-benchmarks depend on vk_system
rather than build part of System source files.

Bug: b/145758253
Change-Id: I9cce59ca90d91601696f6195326b5fd7dde517d5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43690
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>