Benchmarking The Work-In-Progress Spectre/STIBP Code On The Way For Linux 4.20

Due to the performance fallout from the introduction of STIBP for cross-hyperthread Spectre Variant Two mitigation in the Linux 4.20 kernel, the stable Linux kernels are reverting those patches after originally being quickly back-ported to those branches. For Linux 4.20, STIBP on by default for all processes remain in place until the revised STIBP code is ready for merging that is still expected to happen before the stable Linux 4.20 debut in about one month's time. Here are some initial benchmarks of those preliminary improvements.