Parts of testing in GitHub are currently still disabled as they needed changes to work with JDK 8.

This change does fixes for JDK 8 and enables them. Enabled tests include building hotspot in different configurations and also cross building for other architectures.

Notes on modifications:
- optimized jvm-variant does not exist on JDK 8, client variant tested instead
- added support for multilib testing to linux additional (to existing native and cross) as client and minimal variants are only for 32-bits on JDK 8
- arm and s390x are only available as zero variant on JDK 8 so they use this variant (added required libffi to buildroot)
- JDK 8 does not support --with-build-jdk configuration option (not to be confused with boot jdk), so it was removed as well as code preparing build jdk
- dropped dependence of linux additional on linux x64 build (was only needed to get build jdk)
- In case of JDK 8, a bit more persuasion was required to successfully configure cross build. Explicit configuration of libs was required. Also *FLAGS env. variables with sysroot had to be set for configuration to succeed, but they must not remain set for build, otherwise it fails.

Testing:
All test passed in Github CI.