[OpenMP][libomptarget] Allow overriding function that gets ELF symbol info

The OpenMP target's NextGen plugins retrieve symbol information in the ELF image
(i.e., address and size) through the ELF section and ELF symbol objects. However,
the images of CUDA programs compute the address differently from the images of
AMDGPU programs:

  - Address for CUDA symbols: image begin + section's offset + symbol's st_value
  - Address for AMDGPU symbols: image + begin + symbol's st_value

Differential Revision: https://reviews.llvm.org/D138604