Kernel: Fix the return type for syscalls

The Process::Handler type has KResultOr<FlatPtr> as its return type.
Using a different return type with an equally-sized template parameter
sort of works but breaks once that condition is no longer true, e.g.
for KResultOr<int> on x86_64.

Ideally the syscall handlers would also take FlatPtrs as their args
so we can get rid of the reinterpret_cast for the function pointer
but I didn't quite feel like cleaning that up as well.

Kernel/Syscalls/ftruncate.cpp

@@ -9,7 +9,7 @@
 
 namespace Kernel {
 
-KResultOr<int> Process::sys$ftruncate(int fd, Userspace<off_t*> userspace_length)
+KResultOr<FlatPtr> Process::sys$ftruncate(int fd, Userspace<off_t*> userspace_length)
 {
     REQUIRE_PROMISE(stdio);
     off_t length;