[libomptarget][amdcgn] Only add opt/llvm-link dependency if TARGET is available

In some build configurations, the target we depend on is not available for declaring the build dependency.
We only need to declare the build dependency, if the build target is available in the same build.

Fixes the issue raised in https://reviews.llvm.org/D107156#2969862
This patch should go into release/13 together with D108404

Differential Revision: https://reviews.llvm.org/D108868