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.