[libomptarget][amdgcn] Tolerate deadstripped env variable

[libomptarget][amdgcn] Tolerate deadstripped env variable

Discovered by Pushpinder. If the device_environment variable is unused
it can be deadstripped, in which case we should not abort due to it
missing. This change is safe in that a missing symbol which is actually
used can be reported by both linker and loader, and a missing unused
symbol is better deadstripped than left in the image.

Reviewed By: pdhaliwal

Differential Revision: https://reviews.llvm.org/D96329