external/brotli optimization (impossible?)

[enh-google]

https://github.com/google/brotli/commit/f9b8c02673c576a3e807edbf3a9328e9e7af6d7c added basic rv64 support 6 years ago (and covers "is it 64-bit?" and "is it fine with misaligned loads?").

beyond that, there's very little that's really architecture-specific beyond the arm ubfx and rbit instructions --- but afaik there are no rv64 equivalents of those instructions?

i didn't find any architecture-specific optimizations in zopfli.