[compiler-rt] [AArch64][compiler-rt] Add memcpy, memset, memmove, memchr builtins. (PR #77496)

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

The point of this implementation is that it doesn't use NEON instructions, as those are not valid in Streaming-SVE mode. We'll definitely want to optimise these routines by using (streaming-compatible) SVE instructions or by using the MOPS instructions, if available. This PR is merely a starting point to guarantee functional correctness.

It is also possible to use regular memcpy routines, but those will incur a `smstop/smstart` around the call-site when called from a streaming function.

https://github.com/llvm/llvm-project/pull/77496