[HIP] Make __hip_gpubin_handle hidden to avoid being merged across different shared libraries

Different shared libraries contain different fat binary, which is stored in a global variable
__hip_gpubin_handle. Since different compilation units share the same fat binary, this
variable has linkonce linkage. However, it should not be merged across different shared
libraries.

This patch set the visibility of the global variable to be hidden, which will make it invisible
in the shared library, therefore preventing it from being merged.

Differential Revision: https://reviews.llvm.org/D50596

llvm-svn: 340056