Add cmovz/cmovnz support for aarch64.

[codahale]

This passes tests and I believe it works, but I also have the assembly programming experience of your average kindergartner, so a second opinion would be worth getting.

Running cargo rustc --release -- --emit asm on the portable cmovz yields this:

        neg     w8, w0
        orr     w8, w8, w0
        ubfx    x8, x8, #7, #1
        ldr     x9, [x2]
        neg     x10, x8
        and     x9, x9, x10
        sub     x8, x8, #1
        and     x8, x8, x1
        orr     x8, x9, x8
        str     x8, [x2]
        ret

And a naive, branching version of cmovz yields this:

        cbz     x0, LBB1_2
        ret
LBB1_2:
        str     x1, [x2]
        ret

Which makes me think cmp/csel is the right choice.

[tarcieri]

Looks good, thank you!

Can you update the documentation to reflect AArch64 assembly is available as well?

[codahale]

I updated README.md and Cargo.toml.

[tarcieri]

Thank you!