Rust: Move 'libformat_parser' build into libgrust

Addresses #2883.

	contrib/
	* gcc_update (files_and_dependencies): Update for
	'libformat_parser' in libgrust.
	gcc/rust/
	* Make-lang.in (LIBFORMAT_PARSER): Point to 'libformat_parser'
	build in libgrust.
	(%.toml:, $(LIBFORMAT_PARSER):): Remove.
	libgrust/
	* libformat_parser/Makefile.am: New.
	* Makefile.am [!TARGET_LIBRARY] (SUBDIRS): Add 'libformat_parser'.
	* configure.ac: Handle it.
	(TARGET_LIBRARY): New 'AM_CONDITIONAL'.
	* libformat_parser/Makefile.in: Generate.
	* Makefile.in: Regenerate.
	* configure: Likewise.