[libomptarget-nvptx] Test bitcode compiler flags and enable by default

Move all logic related to selecting the bitcode compiler and linker
into a new file and dynamically test required compiler flags. This
also adds -fcuda-rdc for Clang trunk as previously attempted in D44992
which fixes the build.

As a result this change also enables building the library by default
if all prerequisites are met.

Differential Revision: https://reviews.llvm.org/D46901

llvm-svn: 332494

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