rocm: make amdgpu_target sticky (#34591)

The sticky property will prevent clingo from changing the amdgpu_target
to work around conflicts. This is the same behaviour as was adopted for
cuda_arch in 055c9d125d.

lib/spack/spack/build_systems/rocm.py

@@ -132,6 +132,7 @@ class ROCmPackage(PackageBase):
         "amdgpu_target",
         description="AMD GPU architecture",
         values=spack.variant.any_combination_of(*amdgpu_targets),
+        sticky=True,
         when="+rocm",
     )