[druntime] core.int128: Optimize a bit and prepare for more LDC/GDC optimizations

[kinke]

Main change: extract the nested udivmod128_64 helper function as new udivmod overload, and optimize that one via inline asm on x86_64. Handy e.g. here: dlang/phobos#10688 (comment)

[dlang-bot]

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[ibuclaw]

Need tests? Even though the existing 128bit ones already tickle all code paths.