gccrs: Get rid of make builtin macro

This macro is a mess and a helper method is much better for this case.

gcc/rust/ChangeLog:

	* resolve/rust-name-resolver.cc (MKBUILTIN_TYPE): Remove macro.
	(Rib::Rib): Remove `mappings` field.
	(Resolver::generate_builtins): Use `setup_builtin` instead of macro.
	(Resolver::setup_builtin): New function.
	* resolve/rust-name-resolver.h: Declare `setup_builtin`, add FIXME
	comment.