Kernel: Allocate new main thread stack before committing to exec

If the allocation fails (e.g ENOMEM) we want to simply return an error
from sys$execve() and continue executing the current executable.

This patch also moves make_userspace_stack_for_main_thread() out of the
Thread class since it had nothing in particular to do with Thread.

Kernel/Process.h

@@ -426,9 +426,10 @@ public:
         size_t size { 0 };
         FlatPtr program_headers { 0 };
         size_t num_program_headers { 0 };
-        AK::WeakPtr<Region> tls_region;
+        WeakPtr<Region> tls_region;
         size_t tls_size { 0 };
         size_t tls_alignment { 0 };
+        WeakPtr<Region> stack_region;
     };
 
     enum class ShouldAllocateTls {