llvm-amdgpu: Fixes "UnboundLocalError: local variable referenced before assignment" (#22562)

Co-authored-by: Jan Ciesko <jciesko@sandia.gov>

var/spack/repos/builtin/packages/llvm-amdgpu/package.py

@@ -69,6 +69,7 @@ def cmake_args(self):
             compiler = Executable(self.compiler.cc)
             gcc_output = compiler('-print-search-dirs', output=str, error=str)
 
+            gcc_prefix = ""
             for line in gcc_output.splitlines():
                 if line.startswith("install:"):
                     # Get path and strip any whitespace