gdb: make disassembler fprintf callback a static member function

The disassemble_info structure has four callbacks, we have three of
them as static member functions within gdb_disassembler, the fourth is
just a global static function.

However, this fourth callback, is still only used from the
disassemble_info struct, so there's no real reason for its special
handling.

This commit makes fprintf_disasm a static method within
gdb_disassembler.

There should be no user visible changes after this commit.