[AMDGPU][Libomptarget] Rename & move g_executables to private

This patch moves g_executables to private member of Runtime class
and is renamed to HSAExecutables following LLVM naming convention.

This movement required making Runtime::Initialize and Runtime::Finalize
non-static. Verified the correctness of this change by running
libomptarget tests on gfx906.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D102600