[ARM] Improve loop unrolling for Cortex-M

- Set the default runtime unroll count to 4 and use the newly added
  UnrollRemainder option.
- Create loop cost and force unroll for a cost less than 12.
- Disable unrolling on Thumb1 only targets.

Differential Revision: https://reviews.llvm.org/D36134

llvm-svn: 310997