sync std-simd and libstdc++ std::experimental::simd code

[mattkretz]

[mattkretz]

• extract test suite from vir-simd / libstdc++-v3/testsuite/simd
• consider cross-cherry-pick, but if it's too much work just skip it (at least for the tests, that's surely the case)
• check git blame to ensure there's no license issue (non-trivial contributions not from GSI)