[llvm] [CodeGen] Gate rematerializer unit tests on AMDGPU target being enabled (PR #202966)

[Lucas Ramirez via llvm-commits]

lucas-rami wrote:

> Combined with avoiding building in cmake?

AFAIU there is no target-conditional building, tests usually initialize all available targets, then query a specific one and skip the test if it's not available.

https://github.com/llvm/llvm-project/pull/202966