Kernel: Add a random offset to kernel stacks upon syscall entry

When entering the kernel from a syscall, we now insert a small bit of
stack padding after the RegisterDump. This makes kernel stacks less
deterministic across syscalls and may make some bugs harder to exploit.

Inspired by Elena Reshetova's talk on kernel stack exploitation.

Kernel/Arch/i386/CPU.h

@@ -447,6 +447,14 @@ inline void read_tsc(u32& lsw, u32& msw)
                  : "=d"(msw), "=a"(lsw));
 }
 
+inline u64 read_tsc()
+{
+    u32 lsw;
+    u32 msw;
+    read_tsc(lsw, msw);
+    return ((u64)msw << 32) | lsw;
+}
+
 struct Stopwatch {
     union SplitQword {
         struct {