[libomptarget] Apply D110029 to amdgpu

Use enum for execution mode.

This is partly a port from ROCm and partly a port from D110029. Attempted to
make the same choices as ROCm as far as comments etc go to reduce the merge
conflicts.

There is some cleanup warranted here - in particular I like the cuda patch
factoring out the comparisons into named variables - but I'd like to leave
that for a follow up patch, keeping this one minimal.

Reviewed By: carlo.bertolli

Differential Revision: https://reviews.llvm.org/D110845