[Downstream Patch][ARM] Disable inline MEMCPY with LDM/STM instructions

This downstream patch is a performance patch for Cortex-V7M cores.
An attempt was made to upstream this work to the LLVM project but it
wasn't accepted by upstream as they would prefer a different solution.

Downstream issue:#405

Change-Id: I0f30c3da60e3f83835702ddf1be6e6e84b7ebdb8

Author: nasherm