[libomptarget][nfc] Explicitly static function scope shared variables

Summary:
[libomptarget][nfc] Explicitly static function scope shared variables

`__shared__` in CUDA implies static in function scope. See e.g. D.2.1.1
in CUDA_C_Programming_Guide.pdf,
http://developer.download.nvidia.com/compute/DevZone/docs/html/C/doc/

This is surprising for non-cuda developers, see e.g. D73239 where I thought
local variables would be thread local.

Tested by IR diff of libomptarget.bc (no change), running in tree tests,
and binary diff of the nvcc static archives (no significant change).

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D76713