Log Message:
lang/ruby: clean up some macro handling

1. Move some variables from gem-vars.mk to rubyversion.mk:

	GEM_EXTSBASE
	GEM_PATH
	GEM_PLUGINSDIR
	RUBYGEM
	RUBY_GEM_ARCH

2. Drop unused GEM_EXTSDIR_NEEDS_SUBDIR (it is always true).

3. Define bundled module's version as "MODULE_VER".

4. Generate PLIST_SUBST and PRINT_PLIST_AWK from bundled module's name
   if corresponding MODULE_VER is defined.

5. Do not run ${RUBYGEM} for RUYB_GEM_ARCH and RUBY_GEM_VER.

NFCI.