Some Arm __aeabi_ builtin implementations are not ABI Compliant

[parched]

See https://bugs.llvm.org/show_bug.cgi?id=35243.

Some of the __aeabi_mem* functions in src/arm.rs probably can't be written in Rust because they have special ABI requirements.

We should wait and see what the resolution of LLVM ticket is.

[Lokathor]

We could just write them directly in assembly. I did that recently, it wasn't bad at all.