Provide faster memory functions

[Lokathor]

• __agbabi_memset
• __agbabi_memcpy
• possibly others?

These functions are provided by compiler_builtins with weak linkage. They're rather poor by default, so we should provide something better in our assembly runtime code.

[Lokathor]

All the appropriate signatures that we need to match are here: https://github.com/rust-lang/compiler-builtins/blob/master/src/arm.rs

[Lokathor]

https://github.com/felixjones/agbabi/tree/2.0 has Zlib licensed samples of seemingly all the functions.

Probably we just need to bring them in and slot them in our existing folder of assembly stuff. The code isn't so commented, so we might want to add more notes inside each function.