Allow disabling class file version check in class reader to better allow ASM users to work with EA releases of the JVM after version numbers are increased.

Makes a formerly package-private constructor accessible. As many ASM users shade ASM, it is already easily possible to use this constructor by creating a subclass in the org.objectweb.asm package that invokes this constructor with the appropriate access rights but it would be nicer to get a more official way to access the constructor.