Remove redundant environment variable XLA_FLAGS.

[jacobhegna]

Summary

If the flag is not set, the script saved_model_aot_compile.py in tensorflow will
default it to the correct value. However, in TF 2.5, the way the value is set in
TensorFlowCompile.cmake file triggers a build error.

[jacobhegna]

Add --multithreading false to the cmake file.

[mtrofin]

lgtm

llvm/cmake/modules/TensorFlowCompile.cmake

if (EXISTS "${override_header}" AND EXISTS "${override_object}")
     configure_file(${override_object} ${obj_file} COPYONLY)
     message("Using provided header "
       ${hdr_file} " and object "   ${obj_file}
       " files for model " ${model})
   else()
     tfgetmodel(${model} LLVM_ML_MODELS_ABSOLUTE)
     message("Using model at " ${LLVM_ML_MODELS_ABSOLUTE})
     add_custom_command(OUTPUT ${obj_file} ${hdr_file}
-      COMMAND "XLA_FLAGS=\"--xla_cpu_multi_thread_eigen=false\"" ${TENSORFLOW_AOT_COMPILER} aot_compile_cpu
+      COMMAND ${TENSORFLOW_AOT_COMPILER} aot_compile_cpu
+            --multithreading false
             --dir ${LLVM_ML_MODELS_ABSOLUTE}
             --tag_set ${tag_set}
             --signature_def_key ${signature_def_key}
             --output_prefix ${prefix}
             --cpp_class ${cpp_class}
             --target_triple ${LLVM_HOST_TRIPLE}
     )
   endif()