Export memcpy as a public symbol

[LucasSte]

Our own memcpy function is not exported as a public symbol, preventing LLVM from liking it to its memory copy intrinsic function. We should set this up and update our function with this optimization:

https://github.com/solana-labs/compiler-builtins/blob/8be91a9c0b76193b0ac093ec32433d6ca508fe1b/src/mem/mod.rs#L281