gccrs: Handle link_name attribute

gccrs: Handle link_name attribute

gcc/rust/ChangeLog:

	* backend/rust-compile-extern.h: Add includes.
	(CompileExternItem::visit): Use get_link_name.
	(CompileExternItem::get_link_name): New static member function.
	* util/rust-attribute-values.h (Attributes::LINK_NAME): New
	static constexpr member variable.
	* util/rust-attributes.cc (__definitions): New entry for
	LINK_NAME.
	* util/rust-ggc.cc: Include "rust-ast.h".
	(Ident::Ident): Add overload for Rust::Identifier.
	* util/rust-ggc.h (class Identifier): Forward declare.
	(Ident::Ident): Add overload for Rust::Identifier.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/link-name.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>