Cranelift: Introduce memcpy and memset instructions

#### Feature

Introduce instructions that behave like memcpy and memset. These should lower to `repe movsb` and `repe stosb` for memcpy and memset respectively on x86_64 with the ermsb feature. According to https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/arm-a-profile-architecture-developments-2021 there is also an AArch64 extension for this, but I couldn't find more details.

#### Benefit

Using a native instruction reduces instruction cache bloat and may be faster in some cases. It may also help future optimizations with recognizing these operations as such to allow optimizing them away in some cases. This is very important for runtime performance of rust code as rustc generates a lot of unnecessary copies of locals.

#### Implementation

The instructions should take an immediate as size argument and be lowered to native instructions if available, or as libcalls to an external memcpy or memset function if not available.