Update RocksDB for devtoolset 11+

[mike-evolved]

Updates the RocksJava build system to support newer GCC compilers (versions 11 and 12) and fixes some cross-platform build issues.

Main changes:

• Docker build script detects and uses devtoolset 9-12 (newer GCC versions), not just the old ones
• Makefile adds variables so users can customize which Docker images to use for builds
• PowerPC builds target Power8 as the minimum CPU for better compatibility, and won't fail if platform detection doesn't work
• s390x builds switched from Ubuntu 18 to CentOS 7 to match other platforms

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