Clean up our CMake scripts using features introduced between 3.0 and 3.14

[jkoritzinsky]

Now that we're using a consistent modern version of CMake on all platforms, we can significantly clean up our native build scripts.

There's a few things I decided to go fix:

• Remove the CLR_CMAKE_COMPILER variable and use generator expressions instead.
• Remove the directory-based dac.cmake, crossgen.cmake, md_dac.cmake, md_wks.cmake, and md_dbi.cmake files and drive their inclusion/exclusion of various defines via custom target properties and generator expressions.
• Collapse any leaf CMakeLists.txt files that were only leaf nodes because of the above directory-based component definition system.
• Convert PCH generation to be target-specific instead of directory-specific.
• Convert linker options to be added via add_link_options when possible.
• Refactor manual asm preprocessing and compilation (for Windows ARM and ARM64) to a single utility function.
• Refactor manual preprocessing to a single xplat function.
• Use more of CMake's built-in features for setting up our cross-compilation toolchain.

[janvorli]

I am surprised we still use (and have) this file instead of using the shared eng/common/cross/toolchain.cmake. It would be great to start using the shared one (and update it - it is out of date w.r.t. the cross/toolchain.cmake)

[jkoritzinsky]

We can't update the version in eng/common/cross until we update all repos that would use it to run on images with CMake 3.14+. I'd prefer to update our cross/toolchain.cmake file, then update the images used by the various other repos with native toolsets. Then I'd update the version in eng/common. Once that propagates through, I'd remove the local one.

[jkoritzinsky]

@janvorli can you take another review pass when you have a chance?

[janvorli]

LGTM, thank you!