Kernel: Clarify why ftruncate() & pread() are passed off_t const*

I fell into this trap and tried to switch the syscalls to pass by
the `off_t` by register. I think it makes sense to add a clarifying
comment for future readers of the code, so they don't fall into the
same trap. :^)

Kernel/Syscalls/ftruncate.cpp

@@ -9,6 +9,8 @@
 
 namespace Kernel {
 
+// NOTE: The length is passed by pointer because off_t is 64bit,
+// hence it can't be passed by register on 32bit platforms.
 ErrorOr<FlatPtr> Process::sys$ftruncate(int fd, Userspace<off_t const*> userspace_length)
 {
     VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this)