RFR: JDK-8230413: Support Pre JDK 6 class with CDS

[Jiangli Zhou]

Please review the following change that allows archiving
pre-JAVA_6_VERSION classes with -Xverify:none.

webrev: http://cr.openjdk.java.net/~jiangli/8230413/webrev.00/
RFE: https://bugs.openjdk.java.net/browse/JDK-8230413

Currently there are still large number of existing classes (pre-built)
with older class versions (< 50) in real world applications. Those
classes are missing the benefit of archiving. Particularly, in some
use cases, class verification can be safely disabled. For those use
cases, supporting archiving pre JDK 6 classes shows good performance
benefit. We can re-evaluate this support when -Xverify:none is removed
in the future, hopefully the needs for supporting class version < 50
is no longer significant at that time.

This change brings back the pre-JDK-8198849 behavior. Runtime makes
sure the dump-time verification mode must be the same or stronger than
the current mode.

A CSR may be needed for the change. Any thoughts on that?

Tested with jtreg appcds tests.

Best,
Jiangli