[Libomptarget] Change visibility to hidden for device RTL

This patch changes the visibility for all construct in the new device
RTL to be hidden by default. This is done after the changes introduced
in D117806 changed the visibility from being hidden by default for all
device compilations. This asserts that the visibility for the device
runtime library will be hidden except for the internal environment
variable. This is done to aid optimization and linking of the device
library.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D117807