* Added validation to ensure that this script is run on the expected host OS.
* Made it restrict the list of platforms LLVM is built for based on the selected
  target platform.
* Made copy_common_generated_files use LLVM_TARGETS to determine which
  folders to copy.
* Other minor cleanup/refactoring.

Bug: b/152339534
Change-Id: I18c6e658688e487a83eb7b6bbbe4990029d47f8f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46269Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>