fix Issue 12800 - Fibers are broken on Win64

[jblume]

Win64 version of the context switch used the wrong registers for parameters.

https://issues.dlang.org/show_bug.cgi?id=12800

Note that when the example in the bug report is built in VisualD with standard debug settings, it still crashes. But that crash is caused by the default setting of only 4k stack space per Fiber, which isn't enough with all the debug switches VisualD sets. Compiling directly with DMD without any special switches works.

[MartinNowak]

Thanks a lot, please enable the Fiber unittests on Win64 though.
https://github.com/jblume/druntime/blob/master/src/core/thread.d#L4496

[jblume]

I also extended the stack alignment test which originally only ran for Posix, as the Windows 64 calling convention demands alignment to 16 byte as well.

I have only started working with DMD a few days ago, so I hope the unit tests actually ran through correctly on my machine. After inserting an assert(0) they failed as expected, so I have some hope it actually worked.

[MartinNowak]

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