The _GLOBAL_OFFSET_TABLE_ is a linker defined symbol that is placed at some location relative to the .got, .got.plt or .toc section. On some targets such as Arm the correctness of some code sequences using a relocation to _GLOBAL_OFFSET_TABLE_ depend on the value of the symbol being in the linker defined place. Follow the ld.gold example and give a multiple symbol definition error. The ld.bfd behaviour is to ignore the definition in the input object and redefine it, which seems like it could be more surprising.
I've restricted the effect of the change to _GLOBAL_OFFSET_TABLE_ rather than making it universal for all linker defined symbols as there are some existing tests that explicitly redefine some linker defined symbols in input objects citing some previous problem. My justification for special casing _GLOBAL_OFFSET_TABLE_ is that it is the only symbol that the linker uses the value of internally, the use of other linker defined symbols is defined by the input objects only.
A test program to check the behaviour of ld.gold and ld.bfd using the symbol names in addReservedSymbols() showed that ld.bfd would just redefine the symbols and ignore the input definitions. ld.gold would error if there was a non-common definition of the symbol.
fixes pr39587 (turning incorrect output into an error message)