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.

gdb/disasm.h

@@ -82,6 +82,9 @@ private:
      non-memory error.  */
   gdb::optional<CORE_ADDR> m_err_memaddr;
 
+  static int dis_asm_fprintf (void *stream, const char *format, ...)
+    ATTRIBUTE_PRINTF(2,3);
+
   static int dis_asm_read_memory (bfd_vma memaddr, gdb_byte *myaddr,
 				  unsigned int len,
 				  struct disassemble_info *info);