[llvm] [Libomptarget] Rework device initialization and image registration (PR #93844)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Tested this on a heterogenous machine and it passes all the tests and offloads to the separate devices as intended.

@ronlieb @carlobertolli AMD version https://gist.github.com/jhuber6/2c58f940d9406fa3adc94371c7bacdb0. This moved around @carlobertolli's APU handling code, so please look at it to see if it makes sense.

https://github.com/llvm/llvm-project/pull/93844