[libomptarget][amdgpu] Convert an assert to print and offload_fail

[libomptarget][amdgpu] Convert an assert to print and offload_fail

The kernel launched is supposed to be present in the binary, but a not yet
diagnosed bug means it is missing for some of the qmcpack test cases. Changing
from assert to print and offload_fail should help diagnose that and similar bugs.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D102378