Miscellaneous changes to support in-tree builds.

[stellaraccident]

• Conditions find_package hsakmt on the pressence of the target.
• Move finding of clang/llvm-objcopy to a common place and use an in-tree target if available.
• Add options for depending on in-tree device libs.

This is a roll-up of changes that I needed to do in order to build ROCR-Runtime with other parts of the toolkit. In my final arrangement, I am only using the first point, but I believe the second/third are general improvements -- so including those as well.