Use DW_ATE_UTF for the Rust 'char' type

The Rust 'char' type should use the DWARF DW_ATE_UTF encoding.

	gcc/
	* dwarf2out.cc (is_rust): New.
	(base_type_die): Use DW_ATE_UTF for the Rust 'char' type.
	(gen_compile_unit_die): Handle "GNU Rust".

Co-authored-by: Mark Wielaard <mark@klomp.org>
Co-authored-by: Marc Poulhiès <dkm@kataplop.net>