Keep vm->orig_progname alive

	`vm->orig_progname` can be different from `vm->progname` when user
	code assigns to `$0`. While `vm->progname` is kept alive by the
	global table, nothing marked `vm->orig_progname`.

	[Bug #15887]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67721 b2dd03c8-39d4-4d8f-98ff-823fe69b080e