[libc] Add basic wrappers for the backend address spaces

The GPU makes use of different address spaces. We generally work with
global memory, thread private memory, and thread shared memory. This
patch simply adds a few preliminary wrappers to map these concepts to
the numerical values the backend uses. Obviously casts between these
will need to be checked by the user.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D156731