builtins: add it blocks for Thumb-2

Add the missing IT-blocks for Thumb-2 compilation for code paths exercised by
older ARM CPUs.  This should fix the buildbots.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@211035 91177308-0d34-0410-b5e6-96231b3b80d8