[Issue 23614] ImportC: __int128 not supported

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23614

--- Comment #4 from Jacob Carlborg <doob at me.com> ---
Here's the full preprocessor logic:

#if !defined BR_INT128 && !defined BR_UMUL128
#ifdef __SIZEOF_INT128__
#define BR_INT128    1
#elif _M_X64
#define BR_UMUL128   1
#endif
#endif

Then rest of the code checks for "BR_INT128" and "BR_UMUL128". Perhaps "_M_X64"
should be undefined as well.

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