add weak memcpy et al (take 2)

[japaric]

cc @Amanieu

[japaric]

Splitting rlibc into its own crate yields correct __aeabi_* intrinsics

00000000 <__aeabi_memcpy>:
   0:   eafffffe        b       0 <memcpy>

But using this crate still generates a recursive bomb.

[Amanieu]

So is the problem inside the rlibc crate? Is the memcpy function calling itself?

[japaric]

The rlibc disassembly loosk correct.

Interestingly, disabling LTO on this branch doesn't fix the infinite recursion.

[Amanieu]

memcmp should be added as well since it is called by libcore.

[Amanieu]

I don't think an extern crate is necessary here since you're getting the symbols using extern "C".