[WIP] Create NVPTX bitcode libraries for devicelib

[MartinWehking]

Add CMake to emit .bc files when compiling devicelib libraries to bitcode for NVPTX.

Make the driver consume these files.

This is a WIP with many sledgehammer-like additions and many missing bits (e.g. addition of custom commands etc.)

Ultimately, the .bc files will be combined into a single bc library