jtreg AOT tests should not assume library extension of .so

The jtreg AOT tests are written with the assumption that shared libraries have a ".so" file extension. The extension ".so" is hard coded in most of the tests and in supporting java sources such as hotspot/test/cli/jaotc/JaotcTestHelper.java.

     public static final String DEFAULT_LIB_PATH = "./unnamed.so";

JDK-8172670 adds support for Windows and Mac which use different file extensions.

The current jaotc implementation is working around this issue by allowing any file extension to be specified during AOT compilation. This work-around should be removed once this issue is fixed.