[libomptarget][amdgcn] Tolerate deadstripped device_state variable

[libomptarget][amdgcn] Tolerate deadstripped device_state variable

The device_state variable may have been deadstripped. Similar to
device_environment, leave detection of missing but used symbol to loader.

Reviewed By: pdhaliwal

Differential Revision: https://reviews.llvm.org/D96330