Fix ExitStatus on Fuchsia

Fuchsia exit codes don't follow the convention of libc::WEXITSTATUS et
al, and they are 64 bits instead of 32 bits. This gives Fuchsia its own
representation of ExitStatus.

Additionally, the zircon syscall structs were out of date, causing us to
see bogus return codes.

src/libstd/sys/unix/process/mod.rs

@@ -1,5 +1,5 @@
-pub use self::process_common::{Command, ExitStatus, ExitCode, Stdio, StdioPipes};
-pub use self::process_inner::Process;
+pub use self::process_common::{Command, ExitCode, Stdio, StdioPipes};
+pub use self::process_inner::{ExitStatus, Process};
 pub use crate::ffi::OsString as EnvKey;
 
 mod process_common;