Add support for new arm memcpy instructions

Armv8.8/9.3 has FEAT_MOPS which adds memcpy, memmove, and memset
instructions. In addition to adding to core DR's decoder, we need
memtrace-specific work to instrument these: probably by unrolling the
loops like we do for x86 rep string operations.

Locate the core DR decoder and the memtrace handling for x86 REP string operations, which the issue identifies as the model for this work. The change is done when Arm FEAT_MOPS memcpy, memmove, and memset instructions decode correctly and receive the required memtrace instrumentation.

Written by the indexing model from the issue text.