compiler-rt: prefer thumb over ARM

When possible, use Thumb or Thumb-2 over ARM instructions.  This is particularly
important for pure-Thumb environments (e.g. Windows on ARM).  Although, it is
possible to conditionalise this for that target specifically, this is available
on most newer ARM CPUs, and the code remains compatible with older CPUs with no
adverse effects.  It therefore feels better to always prefer Thumb when
possible.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@211032 91177308-0d34-0410-b5e6-96231b3b80d8
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