Move -Os from JVM feature 'minimal' to new feature 'opt-size'

Prior to JDK-8239450, it was possible to explicitly set the JVM feature 'minimal', even when building variant server. The reason for doing this was to get the code in JvmFeatures.gmk that sets JVM_OPTIMIZATION := SIZE (except for a handful of files).

Since this functionality is important in itself, it should be controlled by a separate feature, instead of just as a part of the 'minimal' feature, which signals that the minimal JVM variant is being built.