[Downstream Patch][ARM] Disable inline MEMCPY with LDM/STM instructions (#406)

This downstream patch is a performance patch for Cortex-V7M cores. An
attempt was made to upstream this work to the LLVM project:

llvm/llvm-project#106378

but it wasn't accepted by upstream as they would prefer a different
solution.

Downstream issue:#405

Author: nasherm