[phobos] Can somebody with OSX please reduce the Phobos unittest failure?

[Don Clugston]

We have an OSX-only failure, which I think is a compiler bug.
It's failing in std.bigint, which only imports
std.internal.math.biguintCore and std.internal.math.bignumX86.
There's no dependency on anything else in Phobos, so it ought to be
pretty easy to reduce.
I can't reduce it, because I don't have OSX.

The failing test is this one:
    assert(BigInt(0x1234_5678_9ABC_5A5AL).toLong() == 0x1234_5678_9ABC_5A5AL);

This is blocking a release.