[CMake][compiler-rt] Only set lld as available when it was built

The `-fuse-ld=lld` check might succeed because there's a system lld,
but that lld may be out-of-date which would cause any tests that
attempt to use for LTO fail. This was observed on some of the bots.

Differential Revision: https://reviews.llvm.org/D81629

compiler-rt/CMakeLists.txt

@@ -535,8 +535,6 @@ else()
   set(COMPILER_RT_LLD_PATH ${LLVM_MAIN_SRC_DIR}/../lld)
   if(EXISTS ${COMPILER_RT_LLD_PATH}/ AND LLVM_TOOL_LLD_BUILD)
     set(COMPILER_RT_HAS_LLD TRUE)
-  else()
-    set(COMPILER_RT_HAS_LLD ${COMPILER_RT_HAS_FUSE_LD_LLD_FLAG})
   endif()
 endif()
 pythonize_bool(COMPILER_RT_HAS_LLD)