This moves logic from assembly to Rust and removes the special
case for exit/panic handling, merging it with regular usercall
handling.
Also, this fixes a bug in the exit usercall introduced in a75ae00.
The bug would make regular exits look like panics with high
probability. It would also with some probability leak information
through uncleared registers.
Add `io` and `arch` modules to `std::os::fortanix_sgx`
This PR adds two more (unstable) modules to `std::os::fortanix_sgx` for the `x86_64-fortanix-unknown-sgx` target.
### io
`io` allows conversion between raw file descriptors and Rust types, similar to `std::os::unix::io`.
### arch
`arch` exposes the `ENCLU[EREPORT]` and `ENCLU[EGETKEY]` instructions. The current functions are very likely not going to be the final form of these functions (see also https://github.com/fortanix/rust-sgx/issues/15), but this should be sufficient to enable experimentation in libraries. I tried using the actual types (from the [`sgx-isa` crate](https://crates.io/crates/sgx-isa)) instead of byte arrays, but that would make `std` dependent on the `bitflags` crate which I didn't want to do at this time.
