[libc] Add a support library for GPU utilities

The GPU has many features that can only be accessed through builtin or
intrinsic functions. Furthermore, these functions are unique for each
GPU target. This patch outlines an interface to create a common `libc`
interface to access these. Currently I only implement a function for the
CUDA equivalent of `blockIdx.x`. More will be added in the future.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D148635