Kernel: Add a blunt big process lock.

We can't have multiple threads in the same process running in the kernel
at the same time, so let's have a per-process lock that threads have to
acquire on syscall entry/exit (and yield while blocked.)

Kernel/Process.cpp

@@ -550,6 +550,7 @@ Process::Process(String&& name, uid_t uid, gid_t gid, pid_t ppid, RingLevel ring
     , m_executable(move(executable))
     , m_tty(tty)
     , m_ppid(ppid)
+    , m_big_lock("Big Process Lock")
 {
     dbgprintf("Process: New process PID=%u with name=%s\n", m_pid, m_name.characters());