Various changes

[Amanieu]

• Cleaned up div implementation and added the rest of the unsigned division builtins
• Added integer multiply builtins
• Added shift builtins
• Removed absv
• Disabled no_mangle for tests so we can compare with the native version
• Optimized the ARM asm a bit

[japaric]

Awesome! Looks good to me, but there's an issue around tests:

Quickcheck is biased in its generation of integer values; it only generates values in a small range ([-100, 100]) (cf BurntSushi/quickcheck#119), so these tests won't hit important branches that e.g. deal with overflow. This is why I was generating a u64 from 2 u32s in the udiv* tests. I'm going to land this and we can address the test coverage later in a more general way.

On another note, I re-measured my u64 % u64 program and this PR brings the binary size down to 946 bytes, which is 30 bytes smaller than the compiler-rt.rs version 👍.