rust: Reintroduce TARGET_RUST_CPU_INFO hook

rust: Reintroduce TARGET_RUST_CPU_INFO hook

gcc/ChangeLog:

	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in: Add @node for Rust language and ABI, and document
	TARGET_RUST_CPU_INFO.

gcc/rust/ChangeLog:

	* rust-lang.cc (rust_add_target_info): Remove sorry.
	* rust-session-manager.cc: Replace include of target.h with
	include of tm.h and rust-target.h.
	(Session::init): Call targetrustm.rust_cpu_info.
	* rust-target.def (rust_cpu_info): New hook.
	* rust-target.h (rust_add_target_info): Declare.