Add define-rtl-version.

	Add define-rtl-version.
	* read.scm (-CGEN-RTL-VERSION): Renamed from -CGEN-LANG-VERSION.
	(cgen-rtl-major, cgen-rtl-minor): Similarly renamed.
	(cgen-lang-fixlevel): Delete.
	(-supported-rtl-versions): New variable.
	(-cmd-define-rtl-version): New function.
	(init-reader!): New command define-rtl-version.
	(-cgen): Add rtl version to --version output.
	* doc/rtl.texi: Document rtl version support.
	* cpu/play.cpu: Add define-rtl-version.