[Libomptarget] Fix external visibility for internal variables

After the changes in D117362 made variables declared inside of a target
declare directive visible outside the plugin, some variables inside the
runtime were given visiblity that conflicted with their address space
type. This caused problems when shared or local memory was made
externally visible. This patch fixes this issue by making these
varialbes static within the module, therefore limiting their visibility
to being internal.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D117526