Generate VMVersion::_futures_string lazily

JDK-8283199 shows that _futures_string initialization affects JVM startup. The point measures show it takes about 50 microseconds after JDK-8283199 with existing sysfs file, and about 100 microseconds with non-existing one. The more bullet-proof way would be to avoid generating the _futures_string on startup altogether. It looks like this string is only used during error reporting, and we already read /proc and /sys on those error handling paths.