This silences the compile warnings:
   config/gcn/gcn-protos.h:78:61: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]

and is in line with many other commits that remove 'register'.
I also note that the the definition in config/gcn/gcn.cc already uses
   print_operand_address (FILE *file, rtx mem)
and the "print_operand_address" prototype/definition in other config/*
also don't have 'register' ...

Committed as r13-1385-g7780dc5b2d02785186583fc8eced3c9e3aec4552

Tobias
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