[libc] Support add_object_library for the GPU build

This patch unifies the handling of generating the GPU build targets
between the `add_entrypoint_library` and the `add_object_library`
functions. The `_build_gpu_objects` function will create two targets.
One contains a single object file with several GPU binaries embedded in
it, a so-called fatbinary. The other is a direct compile of the
supported target to be used internally only. This patch pulls out some
of the properties logic so that we can handle both more easily. This
patch also required adding an ovverride  `NO_GPU_BUILD` for cases when
we only want to build the source file as normal.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D144214