[jdk8u-dev] RFR: 8301550: [8u] Enable additional linux build testing in GitHub

[Zdenek Zambersky]

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.

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Commit messages:
 - Enable linux additional build

Changes: https://git.openjdk.org/jdk8u-dev/pull/254/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=254&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301550
  Stats: 63 lines in 1 file changed: 23 ins; 26 del; 14 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/254.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/254/head:pull/254

PR: https://git.openjdk.org/jdk8u-dev/pull/254