[llvm-readobj] - An attempt to fix BB after D92018.

AVR and PPC64 bots reports link errors:
(http://lab.llvm.org:8011/#/builders/112/builds/1522)
(http://lab.llvm.org:8011/#/builders/52/builds/1764)

/tmp/cclOvLx0.s: Assembler messages:
/tmp/cclOvLx0.s:9223: Error: symbol `_ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined
/tmp/cclOvLx0.s:9227: Error: symbol `.L._ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined
/tmp/cclOvLx0.s:10272: Error: symbol `_ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined
/tmp/cclOvLx0.s:10276: Error: symbol `.L._ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined
/tmp/cclOvLx0.s:10285: Error: symbol `_ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined
/tmp/cclOvLx0.s:10289: Error: symbol `.L._ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined

/tmp/ccFJYr6I.s: Assembler messages:
/tmp/ccFJYr6I.s:6284: Error: symbol `_ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined
/tmp/ccFJYr6I.s:7053: Error: symbol `_ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined
/tmp/ccFJYr6I.s:7093: Error: symbol `_ZN4llvm12function_refIFvvEE11callback_fnIUlvE2_EEvl' is already defined

I *guess* the reason might be the default lambda argument. I've removed it.