Fix symbol_table::change_decl_assembler_name when DECL_RTL is already computed

Fix symbol_table::change_decl_assembler_name when DECL_RTL is already computed

while working on patch assigning unique names to static symbols I noticed that
fortran symbols are not renamed since the frontend calls make_decl_rtl.  This
gets DECL_ASSEMBBLER_NAME and DECL_RTL out of sync. I think we can drop that
call, but it is also good idea to avoid this inconsistence, so this patch makes
symbol_table::change_decl_assembler_name to recompute DECL_RTL in this case.

gcc/ChangeLog:

	* symtab.cc (symbol_table::change_decl_assembler_name): Recompute DECL_RTL
	in case it is already computed.