Similar to #2664 but for return types.

Syscalls can return various types (`int`, `size_t`, pointers, etc). We
probably don't want to return the wrong type, which the plugin may
interpret incorrectly. For example if a syscall is supposed to return an
`int`, we probably don't want to return a `u64::MAX` from our syscall
handler.

Currently our syscall handlers return a `SysCallReg`, which is a 64-bit
union of various types, and there is no type checking to make sure our
syscall handler is returning a valid type. This PR allows our syscall
handlers to specify an explicit return type (ex: `libc::c_int` instead
of `SysCallReg`).

Only the "sched.h" syscall handlers have been updated to use this. I
didn't bother with the others. We can update them later if we want to,
but for now this change allows us to use this for new syscall handlers
that we write in the future.

(Related #2658.)

Before:

```rust
pub fn sched_yield(_ctx: &mut SyscallContext) -> SyscallResult {
    Ok(0.into())
}
```

After:

```rust
pub fn sched_yield(_ctx: &mut SyscallContext) -> Result<libc::c_int, SyscallError> {
    Ok(0)
}
```