fixdfti test fails if float generation gets fixed

[est31]

In build.rs, there is code to use the random generator to create floats.

It has a little bug though in the way it determines the exponent. The exponent is first &ed with the mask, and then shifted. The issue is though, the mask is already assuming a shifted value. So we end up with the expression always being 0.

Instead it should be first shifted, and then &ed with the mask, like ((exponent << (SIGNIFICAND_BITS -1)) & EXPONENT_MASK) |.

When doing this though, I discovered that the fixdfti test fails. I first thought it was a conversion my PR added, but its in fact already present.

The error looks like:

thread 'fixdfti' panicked at 'assertion failed: `(left == right)` (left: `((5179139571476070400,), -170141183460469231731687303715884105728)`, right: `((5179139571476070400,), 170141183460469231731687303715884105727)`)'

[est31]

This might be related to rust-lang/rust#10184

[AaronKutch]

This has been fixed by #397