[OpenMP] Support for global variables when in auto zero-copy. (#80876)

When building without unified_shared_memory, global variables are
declared in the device binary and allocated upon loading onto GPU
memory. However, when running in zero-copy mode (same as with
unified_shared_memory) D2H and H2D copies for mapped local and global
variables are turned off. This patch turns back on H2D and D2H copies
when they refer to global variables, enabling an application built
without unified_shared_memory to work correctly with global variables
when run under automatic zero-copy.

Co-authored-by: Doru Bercea <doru.bercea@amd.com>
Co-authored-by: Jan-Patrick Lehr <janpatrick.lehr@amd.com>