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_5@67761 b2dd03c8-39d4-4d8f-98ff-823fe69b080e