[WIP] Implement open

[mrhota]

We depend on rust-lang/libc now because it has a larger set of types and
constants than we do right now, and it includes a nice bit of
infrastructure to support a wider array of architectures.

We retained a small number of constants not currently included in libc.

Requires a rather substantial amount of pthreads work (cancelation, for example) before tests will pass.

[anp]

Is there a reason that the tests can't pass without work on pthreads? I'm very hesitant to merge anything that would leave the master tests non-green.

[mrhota]

@dikaiosune definitely do not merge this. I'm still working on this, and I will push up additional commits when ready. I just created this PR more-or-less to indicate what I'm working on

[mrhota]

@dikaiosune btw, I'm pseudo-blocked here until I get global_asm!() implemented in rustc. See rust-lang/rust#35119

That macro would make adding platform-specific assembly trivial: no build script hacks, no #[naked] hacks, just global_asm!(include_str!("path/to/syscall_cp.s")) or whatever.