Kernel: Fix intermittent assertion failure in sys$exec()

While setting up the main thread stack for a new process, we'd incur
some zero-fill page faults. This was to be expected, since we allocate
a huge stack but lazily populate it with physical pages.

The problem is that page fault handlers may enable interrupts in order
to grab a VMObject lock (or to page in from an inode.)

During exec(), a process is reorganizing itself and will be in a very
unrunnable state if the scheduler should interrupt it and then later
ask it to run again. Which is exactly what happens if the process gets
pre-empted while the new stack's zero-fill page fault grabs the lock.

This patch fixes the issue by creating new main thread stacks before
disabling interrupts and going into the critical part of exec().

Kernel/Thread.h

@@ -351,7 +351,8 @@ public:
 
     void set_default_signal_dispositions();
     void push_value_on_stack(u32);
-    void make_userspace_stack_for_main_thread(Vector<String> arguments, Vector<String> environment);
+
+    u32 make_userspace_stack_for_main_thread(Vector<String> arguments, Vector<String> environment);
 
     void make_thread_specific_region(Badge<Process>);