[CUDA] Add option to generate relocatable device code

[CUDA] Add option to generate relocatable device code

As a first step, pass '-c/--compile-only' to ptxas so that it
doesn't complain about references to external function. This
will successfully generate object files, but they won't work
at runtime because the registration routines need to adapted.

Differential Revision: https://reviews.llvm.org/D42921

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@324878 91177308-0d34-0410-b5e6-96231b3b80d8