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:

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

After:

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