fix bigfloat itrunc error

[simonbyrne]

Currently itrunc(2.7) == 3. This fixes it, and some errors in the Ziggurat tables that arose. See #9133.

[tkelman]

good catch, thanks for splitting this out

[simonbyrne]

I think the to_mpfr function might be 0.4 only, but you can just replace that call by a constant 1.

[tkelman]

to_mpfr exists in 0.3.3.

  | | |_| | | | (_| |  |  Version 0.3.3 (2014-11-23 20:19 UTC)
 _/ |\__'_|_|_|\__'_|  |
|__/                   |  i686-w64-mingw32

julia> Base.MPFR.to_mpfr(Base.RoundToZero)
1

[tkelman]

Backported in d7c9518 - I forgot the --date though, whoops. Had to fiddle with it to get it to apply via curl https://github.com/JuliaLang/julia/pull/9138.patch | sed 's/UInt/Uint/g' | git apply --whitespace=fix.