PR21518: Use unsigned arithmetic for trapping add/sub functions.

The code in {add,sub}v.i3 routines does not trap when it should, because
it performs the actual add/subtract operation in signed arithmetic,
rather than unsigned.

Patch by Francois-Xavie Coudert!

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@221826 91177308-0d34-0410-b5e6-96231b3b80d8