[libomptarget] Load images in order of registration

This makes sure that images are loaded in the order in which they are registered with libomptarget.

If a target can load multiple images and these images depend on each other (for example if one image contains the programs target regions and one image contains library code), then the order in which images are loaded can be important for symbol resolution (for example, in the VE plugin).
In this case: because the same code exist in the host binaries, the order in which the host linker loads them (which is also the order in which images are registered with libomptarget) is the order in which the images have to be loaded onto the device.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D95530