[NFC][libclc] Add missing evaluation for variable ${tool}_target (#163540)

The variabls should be evaluated before checking for empty.

Author: wenju-he

libclc/CMakeLists.txt

@@ -124,7 +124,7 @@ if( EXISTS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR} )
 endif()
 
 foreach( tool IN ITEMS clang opt llvm-as llvm-link )
-  if( NOT EXISTS "${${tool}_exe}" AND "${tool}_target" STREQUAL "" )
+  if( NOT EXISTS "${${tool}_exe}" AND "${${tool}_target}" STREQUAL "" )
     message( FATAL_ERROR "libclc toolchain incomplete - missing tool ${tool}!" )
   endif()
 endforeach()