[libc][mem*] Introduce Sized/Backends for new mem framework

This patch is a subpart of D125768 intented to make the review easier.

The `SizedOp` struct represents operations to be performed on a certain number of bytes.
It is responsible for breaking them down into platform types and forwarded to the `Backend`.

The `Backend` struct represents a lower level abstraction that works only on types (`uint8_t`, `__m128i`, ...).
It is similar to instruction selection.

Differential Revision: https://reviews.llvm.org/D126768