[Issue 21267] Make std.complex work with -betterC

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21267

--- Comment #1 from Nathan S. <n8sh.secondary at hotmail.com> ---
WIP: https://github.com/dlang/phobos/pull/7643

>on some platforms may be calculated in double precision
>even when T is an extended-precision real.

This might be a showstopper. The two culprits here are version
(CRuntime_Microsoft) and version (CRuntime_UClibc) whose atan2, coshl, expl,
hypotl, logl, powl, and sinhl are effectively just aliases of the
double-precision versions of those functions.

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