rust: Add skeleton support and documentation for targetrustm hooks.

rust: Add skeleton support and documentation for targetrustm hooks.

gcc/ChangeLog:

	* Makefile.in (tm_rust_file_list, tm_rust_include_list, TM_RUST_H,
	RUST_TARGET_DEF, RUST_TARGET_H, RUST_TARGET_OBJS): New variables.
	(tm_rust.h, cs-tm_rust.h, default-rust.o,
	rust/rust-target-hooks-def.h, s-rust-target-hooks-def-h): New rules.
	(s-tm-texi): Also check timestamp on rust-target.def.
	(generated_files): Add TM_RUST_H and rust-target-hooks-def.h.
	(build/genhooks.o): Also depend on RUST_TARGET_DEF.
	* config.gcc (tm_rust_file, rust_target_objs, target_has_targetrustm):
	New variables.
	* configure: Regenerate.
	* configure.ac (tm_rust_file_list, tm_rust_include_list,
	rust_target_objs): Add substitutes.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in (targetrustm): Document.
	(target_has_targetrustm): Document.
	* genhooks.cc: Include rust/rust-target.def.
	* config/default-rust.cc: New file.

gcc/rust/ChangeLog:

	* rust-target-def.h: New file.
	* rust-target.def: New file.
	* rust-target.h: New file.