Fix float->uint conversion for inputs less than 0

Summary:
The spec for these functions says that they should return 0 in this case but
this regressed in r234148. That revision essentially delegates the conversion
to the hardware, but that has different behavior on different platforms (e.g.
it is wrong on x86).

Also fix a typo in the name of __fixunsdfti

Test Plan: The existing unit tests now pass

Reviewers: joerg, howard.hinnant

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9305

llvm-svn: 236319